Phantomjs Headless Runner

Replace jasmine-headles-webkit (which is no longer maintained) runner with phantomjs.

This allows for *full* integration with the rails asset pipeline and complete parity between the in-browser and headless tests.

### spec:javascript rake task
```
# execute full test suite
$ bundle exec rake spec:javascript

# filter testsuite to only execute particular test
$ bundle exec rake spec:javascript SPEC=my_test
```