Switch from PhamtomJS to Firefox

[jhamrick]

Fixes #935

Note that unfortunately, the Firefox webdriver does not support capturing logs from javascript. This is annoying, but not a dealbreaker.

In making this change I uncovered a bunch of race conditions in the javascript tests, which is good I guess!

[jhamrick]

So this has turned out to be way more annoying of a change than I anticipated. In particular, it seems the most annoying thing is that Firefox appears to leak memory as more tests are run. For test_formgrade.py, this is particularly bad since there are a lot of tests and before it even gets halfway through all the tests it is eating up 1GB of memory. I modified the browser fixture so that it closes and reopens Firefox after every test which forces the memory to be freed, but then this makes the tests take 2x as long to run 😞

There are also still some tests that are flaky, like test_load_live_notebook and test_next_prev_assignments in test_formgrade.py. I can't seem to reproduce the failure of test_load_live_notebook locally, and while I can sometimes reproduce test_next_prev_assignments, I cannot determine why it is failing. I suspect there is a problem with the keypresses being sent before the javascript is fully loaded, but Firefox can't capture javascript logs so I cannot isolate the problem 😠