Declare `coverage` dependency (again) for tests.

Travis is not happy without `coverage`. It would be nice if we could
such requirements before commits. Maybe `tox` is the way to go then?
On the other hand it is nice to have different builds in travis.

setup.py

@@ -34,6 +34,7 @@ install_requires = [
 
 tests_require = [
     'pytest >= 2.0.3',
+    'coverage',
     ]
 
 docs_require = [