Added `rvm get stable --auto-dotfiles` to .travis.yml

Added `rvm get stable --auto-dotfiles` to the .travis.yml's before_install.

Travis is currently reporting this:
~~~
$ rvm use default --install --binary --fuzzy
Warning! PATH is not properly set up, '/Users/travis/.rvm/gems/ruby-2.0.0-p247/bin' is not at first place,
         usually this is caused by shell initialization files - check them for 'PATH=...' entries,
         it might also help to re-add RVM to your dotfiles: 'rvm get stable --auto-dotfiles',
         to fix temporarily in this shell session run: 'rvm use ruby-2.0.0-p247'.
~~~

To which `rvm get stable --auto-dotfiles` seems an obvious solution (also suggested here: http://rvm.io/support/faq ).