Migrating .rvmrc file to .ruby-version

new RVM gives the following warn when using .rvmrc files:
```
You are using '.rvmrc', it requires trusting, it is slower and it is not compatible with other ruby managers,
you can switch to '.ruby-version' using 'rvm rvmrc to [.]ruby-version'
or ignore this warnings with 'rvm rvmrc warning ignore /Users/arthurnn/dev/mongoid/.rvmrc',
'.rvmrc' will continue to be the default project file in RVM 1 and RVM 2,
to ignore the warning for all files run 'rvm rvmrc warning ignore all.rvmrcs'.
```

Migrating rvmrc files to ruby-[gemset|version]