Allow for locally chosen rubies

By ignoring .versions.conf, we can allow users pick their own versions
of ruby without having to jump through a bunch of hoops on keeping
.ruby-version straight between local and remote branches.
typo
Add note about rbenv for rvm .versions.conf local override
Land #4136, @todb-r7's fix to allow use of other ruby versions

Only rbenv and rvm have been tested.

For rbenv:

  rbenv shell <VERSION>

For rvm:

  rvm --create --versions-conf use <VERSION>@metasploit-framework