Travis js

This is close. One of the two jobs passed. Oddly, 2.7 passed and 3.3 errored. The version of Python should be irrelevant to `npm`; maybe this is a fluke. I will re-run the build on this same commit to see.

@aflaxman, can you take a look at the log [here](https://travis-ci.org/danielballan/mpld3/jobs/23137989) and see if you can divine what went wrong??