Dedicated inspect implementation for regexes. Fix #2250

We no longer share the implementation with BitString because regex
literals have slightly different escaping rules
Do not install rebar bundled with elixir
Change help heading on mix
Ensure doctests failures are properly tested
Remove unused reduce/3 from map
Merge pull request #2320 from alco/regex-literal

Draft implementation of custom inspect algorithm for regexes
Update CHANGELOG