Add .rvmrc to .gitignore but supply a sample file

* ignoring .rvmrc so people can feel to use their own and not
  accidentally commit it
* added a .rvmrc.sample file for people to get started
* fixes #608
* [ci skip]
Merge pull request #612 from PragTob/GitignoreRVMRC

Add .rvmrc to .gitignore but supply a sample file