Log Message:
Update devel/rebar to 2.6.3.

- rebar/610: qc: lift experimental notice
- rebar/611: port_compiler: consistently format default_env/0
- rebar/612: Introduce and use compat random number module
- rebar/614: Add an additional check for the existence of a 'rebar.beam' file
- rebar/615: port_compiler: generate clang compilation db
- rebar/616: port_compiler: clean up compile_each()
- rebar/617: port_compiler: auto-select C++ specific link template