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
This code will eventually be in the public gem, so referencing a link to the private gem is not good. I would either include more detailed information that is present in the issue, or simply remove this reference.