Use https://rubygems.org and not :rubygems

Since the security breach it's better to keep this stuff seriously.
Also ignore .ruby-version

This is the modern alternative to .rvmrc and .rbenv-version.
Merge pull request #108 from tilsammans/use-secure-rubygems

Use https://rubygems.org and not :rubygems
Merge pull request #109 from tilsammans/ruby-version

Also ignore .ruby-version