Rbenv default gems

@mtylty @kennyadsl: This pull request add https://github.com/sstephenson/rbenv-default-gems and some changes to make it work. Now we have a single file where we can put all our default gems https://github.com/nebulab/laptop/commit/6720018a8d5bbcac8cb4303a233d807a68572ff6 and thanks to rbenv-default-gems every time we install a new ruby version we have the default gems for free. The bad side is that it's don't work with already installed ruby.