Perform general housekeeping tasks

A clean home is a happy home. This pull request doesn't change the functionality of the application. Besides tiding up the source code a bit, I'd like to suggest moving away from the `.rvmrc` file to the `.ruby-version` and `.ruby-gemset` files. They offer the following benefits:

- It's much safer
- It's compatible with other Ruby version managers
- It's not scary (users don't need to accept the initial warning message)

The only drawback is that those files are not compatible with older versions of RVM, which probably shouldn't be used anyway. Any thoughts?