Log Message:
Update devel/rebar to 2.6.0. Add proper Github distfile support.

Changes since 2.5.1:

rebar/203: Pluggable proto compilers gpb
rebar/273: Use target_dir as source of new version in generate-appups
rebar/293: Check C source dependencies in needs_compile
rebar/305: Fix compiler invocation on multiarch Linux
rebar/322: Treat vsn mismatch as warning if -k/--keep-going
rebar/336: Add details on Dialyzer with "make check"
rebar/337: Implement eflame -p/--profile support
rebar/338: Processing .app.src.script expects a single value to be returned.
rebar/344: Manually clean up paths.
rebar/351: fish shell completions for rebar
rebar/352: Add typer target (rebase of #309)
rebar/354: compiler respects 'keep_going' flag
rebar/355: Fix 'make build_plt'
rebar/356: Fix minor typo in CONTRIBUTING.md
rebar/360: Minor follow-up fixes for #293
rebar/368: Escape more characters in path (fix #367)
rebar/371: Fix cover print truncation when coverage is 100%
rebar/372: Implement eval command via nodetool
rebar/376: Remove check adding ebin to path for edoc target
rebar/378: deps: fix delete-deps if deps_dir ends with dot
rebar/382: Adapt dialyzer_reference to ba466e2d changes
rebar/385: Fix Dialyzer warning introduced in 0caf047f
rebar/386: Recompile .proto files with gpb also with prefix/suffix
rebar/386: Recompile .proto files with gpb also with prefix/suffix
rebar/386: Recompile .proto files with gpb also with prefix/suffix
rebar/399: Increase the timeout for the inttest/proto_gpb
rebar/400: add Emacs/vi header to non-dummy test modules
rebar/403: Update reference to installation notes
rebar/404: Consistently format export attributes
rebar/405: Generate cachegrind file if erlgrind is available
rebar/406: Fix deps path check in rebar_ct:collect_glob/3
rebar/408: Fix/remove duplicate console message
rebar/411: Fix app.config argument passing when using ct_run (take 2)
rebar/413: Revert "Merge pull request #386 ...
           gpb-recompilation...detection"
rebar/418: Fix #415 (reltool vsn check)
rebar/420: Mock gpb and protobuffs, in inttest, replacing external
           dependencies
rebar/421: inttest/ct3: fix travis-ci breakage
rebar/424: Gpb recompilation detection (using base compiler)
rebar/425: Copy instead of rsync gpb and protobuffs inttest mocks
rebar/426: Fixed #133. Release upgrade now handle long and short names
           properly.
rebar/428: Remove a git url in gpb and protobuffs inttest configs
rebar/430: Exit with proper status code if 'eval' fails
rebar/432: Document recursive_cmds in -r help string as well
rebar/433: inttest/ct3: fix overlong line
rebar/437: Fix .app.src.script bug introduced in b44b4f4
rebar/440: Delete obsolete file
rebar/442: .travis.yml: print information before running script
rebar/444: Fix incorrect "not an app dir" warning
rebar/445: rebar doesn't respect the order of erl_first_files given
           in the rebar.conf file
rebar/447: Support custom protobuf directory
rebar/449: Support .appup.src files
rebar/452: added 'shell' command to bash-completion
rebar/456: Generate json output from cover
rebar/458: Change env var delimiter to match non-word
rebar/459: Remove -m64 flag.
rebar/460: Add Dialyzer plugin
rebar/461: Fix OTP .appup.src processing on empty lists
rebar/463: Print more info when profiling
rebar/466: Improve test targets in Makefile
rebar/467: Refactor logic and optimizations
           in rebar_erlc_compiler:doterl_compile/4
rebar/469: Fix dialyzer warnings
rebar/470: Fix whitespace errors
rebar/471: Fix whitespace errors
rebar/475: Adapt to 18.x time api changes
rebar/477: Enable parse transformations in rebar config
rebar/478: bootstrap: better warning fix (Thanks James Fish)
rebar/482: Windows runner - CD to node root
rebar/484: /me added to THANKS
rebar/485: avoid pre-compile time errors in expand_include_lib_path
rebar/487: erlc: fix recently introduced whitespace errors
rebar/489: Fix crash on failed build console output
rebar/494: Update line number of allowed dialyzer error
rebar/496: Update PLT detail for make build_plt
rebar/499: Try one more location for the OTP_VERSION file
rebar/500: Makefile: clean only .rebar/erlcinfo
rebar/502: Don't crash if missing OTP_VERSION file (fix #350)
rebar/504: Drop shared PLTs support and change PLT name to .plt
rebar/505: rebar_utils: explain pdict use
rebar/508: rebar_core: add missing newline in log msg
rebar/509: erlc: do not crash if dep file cannot be found
rebar/510: Fix 'make deps'
rebar/511: Add and use memoization server
rebar/512: rmemo: properly handle unsupported call