Use travis to exclude unwanted files from pull requests

[Richard Littauer]

In our Contributing doc:

Please don't include updated versions of rickshaw.js and rickshaw.min.js. Just changes to the source files will suffice.

Perhaps we could use CI to automatically find out if these two files have been added in a Pull Request. If they have, we can fail the tests, and ask that they be removed or cherry-pick around them when merging.