Improve test coverage for Api class
Improve tests structure and test coverage
Improve test suite for Swift_Transport_PostmarkTransport
Improve .travis.yml structure and order
Fix wrong URI test to work both locally and on Travis
Remove test for not receiving a response from Postmark

The test was unreliable or very slow due to different environments.
We would be able to test it better through mocking when we change the way
we determine the send URI.
Merge pull request #10 from OpenBuildings/test-coverage

Improve tests structure and test coverage