Travis integration

This will add Travis integration to your repo. If you are not familiar with Travis, every time you add a new commit, Travis will spin up a new virtual machine, install R, all of your package dependencies and attempt to install this package. If all goes well, it returns a green 'build passing' badge to your README file. If it fails, the badge will turn red, and the person who made the latest commit (that broke things) will get an email. This will catch problems early. 

I made a note in #7