create `~/.rubies` rubie dir on install

This is the dir where rb expects ruby versions to be installed, why
make the user manually create it?  Plus, I want to move the default
ruby version location into this dir so it is becoming more important
to rb.

**Note**: I chose to not rm this dir on uninstall b/c I don't want
to remove all the user's rubies in the process. :)

Related to #40.

@jcredding ready for review.