Remove rbx from allowed failures

[ci skip]
MSP-11103

Timeout issue is fixed
install CONTRIBUTING.md

MSP-11103

Generate CONTRIBUTING.md that is specific to pre-1.0.0 and 1.0.0+.
Remove duplicate `metasploit-version install` from feature

MSP-11103
install RELEASING.md

MSP-11103

Based on advice from @dblockdotorg's Gogaruco 2014 talk
(http://t.co/Gj05HdqsgC), add a RELEASING.md guide for the release steps
and remove those steps from CONTRIBUTING.md.