Need phantomjs tests

[ljharb]

See enzymejs/enzyme#2439 - basically, in phantomjs, the Reflect.apply logic broke without 579179e.

To prevent this from happening again, we need to set up automated tests in phantomjs.

I've gotten both karma and testling to run locally; however, neither is producing the proper exit code to match the tests passing/failing. I've looked into karma-tape/karma-tape-reporter, as example.

I've also come up with an approach to ensure that phantomjs-prebuilt can live in package.json without breaking node 0.8's CI, by making a subpackage that adds it as an optional dep.