Port testing to phantomjs/travisci

I had to make a couple hacks in order to make all the tests pass. In particular, I still haven't resolved the phantomjs rendering problem referenced in [this issue](https://github.com/tdhock/animint/issues/21). I also had to slightly modify test-rect-size.R because test will hang if stroke-width is exactly 0. 

Note that you'll want to change `cpsievert/animint` to `tdhock/animint` in the README and .travis.yml files before merging. You'll also want to get a TravisCI account and "activate" tdhock/animint