Support for IE7, IE8, and PhantomJS 1.x is an albatross.

By dropping support for these environments, we should:

1. Reduce code complexity by removing handrolled shims.
2. Reduce CI failures (hopefully!)
3. Reduce number of edge cases in tests
4. Reduce size of build matrix

Unfortunately I have no metrics on how many use Mocha for these environments.

Please speak up if you are writing new tests for these environments with Mocha, and don't want to see support dropped. I'm willing to entertain a good argument against!