Mockups for Travis

[whoophee]

Currently, tests for Travis use live Steam WebAPI.

This also means that separate mockups have to be generated to accommodate forked pull requests (D2_API_KEY environment variable can't be accessed when building forked pulls).

However, the Steam WebAPI is pretty poorly drawn, in that the responses are subject to change, without any notice. If static mockups are used, we'd have no idea even if the WebAPI was modified.