Switch to browserstack-runner

[localjo]

Right now, it's hard to keep the browsers array in test/selenium-driver.js up to date with the previous versions of our targeted browsers. Switching to browserstack-runner is a potential solution, since it provides [browser]_previous as a way of selecting the previous version of a browser automatically.

There are two downsides that I'm aware of in switching;

First, I don't think browserstack-runner will run the Selenium webdriver tests, but those tests can probably be replaced with some Mocha tests and run programmatically instead of simulating user action. So, the blocker in that aspect is writing some new tests that cover the same things that the Selenium tests cover.

Second, I was running into some issues with browserstack-runner timing out when running tests on certain browsers. We'll need to find a solution in order to run our tests in all of our target browsers. There's an open issue here: browserstack/browserstack-runner#94