Adds dependencies on the mock library from pypi/debian

Dependency on mock was silently introduced in #152 
If that PR built in travis it was probably the result of mock being pre-installed in the test environment, since no dependency on it was added in the PR. 
At first I was thinking about removing the dependency (I like to keep dependencies on non-standard-library modules to a minimum), but since it has nice features, is only used in tests and has been incorporporated into the standard library as of Python 3.3 (unittest.mock) I'm adding it as a debian build dependency and in travis config files as well.

Btw, what's the difference between tox.ini and .travis.yml and why do we need both?