Add a before_all check

The use of `before(:all)` in RSpec tests is almost always unnecessary and often harmful. It can introduce side-effects into the rest of your test suite, and tracking down a bug caused by an improper use of `before(:all)` can be incredibly time-consuming. This patch adds a `before_all` check which checks for instances of `before(:all)` (as well as `before :all`, etc.) in your Ruby files.