Special-cases `RSpec::Mocks::MockExpectationError`

Special-cases `RSpec::Mocks::MockExpectationError`
to remove interference with RSpec testing. Admittedly, RSpec should not be relying on the exception
mechanism to ensure proper results and should instead use some sort of out-of-band
mechanism.

Addresses issue https://github.com/fiksu/af/issues/19
Very good idea. Done!