Switch away from PhantomJS

[lavelle]

Currently all the unit tests run in PhantomJS. It works, but it's running a very old Webkit build, which means constantly polyfilling functions for it. So far in the project I have had to polyfill:

• Function.bind
• ArrayBuffer.slice
• Number.isNaN
• The AWS SDK's HTTP request object

And if we want to use the new native ArrayBuffer to string conversion library we will need to Polyfill TextDecoder too.

It's a waste of time constantly trying to work out whether an error is due to a bug in the code or a missing feature in PhantomJS, so we should switch over to running the tests in Chrome, for a consistent environment. The best way to automate this seems to be Karma.

[lavelle]

Looks like we will still have to use Firefox as Travis CI only supports that, not Chrome. This is still much better than PhantomJS though, and should have all these modern features we need.