Log Message:
Update devel/rebar to 2.6.1.

- Add license to relnotes tool per reviewer request
- Generate reserved file for erlc regression test
- Fixed version of #451
- rebar_utils: fix comment
- Fixed version of #395
- completion: add libid= to bash and zsh scripts
- Allow behaviors defined inside xref_extra_path
- Add regression test for covered common test
- Fix eunit test on OTP18
- Fix #536
- Add erl_first_files in eunit_first_files and qc_first_files
- Complete port compiler help string
- rmemo: use a better R13 check (Reported-by: Stavros Aronis)
- Fix #544
- Added report and verbose options for LFE compile.