Fix TestPreferredTranslatorsFunction test call

Fixes the test for the call to `/account/preferred_translators` that was previously broken. 

This uses mock library to mock request to Gengo API instead of actually calling it. This allows us to test that this library is working correctly, without also testing the Gengo API at the same time. 

Ideally, we should change all the tests to use this method, so that tests can run in seconds, not minutes and anyone can run the tests offline without hitting Gengo's servers.