Upgrade rspec to 3.0.0

Used [transpec](https://github.com/yujinakayama/transpec) to convert the rspec syntax from 2.14 to 3.0.0.

It worked well, the only flaw being that `subject.stub(test_unit?: false, rspec?: true, bundler?: false)` could not be converted automatically. I opted to split this into three seperate `allow` statements.