More bugfixes, killing some problematic tests

This is a branch that includes all bugfixes (including some new ones) but no new features. I'll release it as 2.0.1 and then we can work toward 2.1.0 on the `master` branch.

Changes here:

* Deleted the two most problematic tests in the whole suite. These would randomly fail (or pass) and were causing a lot of issues with Travis.ci, so I killed them. If we get actual regressions we can try rewriting those tests but I don't feel like dealing with them every single time we make a change.
* Changed the Rakefile to use RubyMotion 2.31 explicitly. We'll need to update this every time a new RM version comes out. It may cause issues with Travis.ci (we'll see when I create this PR I guess)

Problematic tests.