Move coverage configuration to .coverage

Introduce a .coveragerc file to omit multiple files from test coverage
such as django's migrations, env and the tests itself.

.coveragerc

@@ -0,0 +1,8 @@
+# .coveragerc to control coverage.py
+
+[run]
+omit =
+	**/migrations/*.py
+	**/tests.py
+	env*
+	/home/travis/virtualenv*

.travis.yml

@@ -5,7 +5,7 @@ install: "pip install -r requirements.txt && pip install coveralls"
 
 script:
   - python manage.py collectstatic --noinput
-  - coverage run --omit='env*' --source='.' manage.py test
+  - coverage run --rcfile .coveragerc manage.py test
 
 after_success:
   - coveralls