Don't ignore .rvmrc

We shouldn't be encouraging people to ignore their .rvmrc in a Rails
project. It's a good idea to ignore it (along with .rbenv-version, and 
the others) in a in a gem which may be used under many ruby versions,
but Rails apps usually target a single version, to avoid nasty surprises
when deploying.

Rails.gitignore

@@ -3,7 +3,6 @@
 .sass-cache
 capybara-*.html
 .rspec
-.rvmrc
 /.bundle
 /vendor/bundle
 /log/*