Easily prefer different 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.

## Verification

- [ ] `rvm install 2.1.4` to get Ruby 2.1.4.
- [ ] In the root of the metasploit-framework checkout, run the command, `rvm --create --versions-conf use 2.1.4@metasploit-framework`
- [ ] `pushd ..; popd` to change directories and back again.
- [ ] Notice that you are now using 2.1.4
- [ ] Notice that `git status` is not complaining about untracked files.