Actually add source directory.

.travis.yml

@@ -31,11 +31,11 @@ script:
   - pip install coverage codecov
 
   # Test without c extensions
-  - python -m coverage run -p -m $TEST_RUNNER dulwich.tests.test_suite
+  - python -m coverage run -p --source=dulwich -m $TEST_RUNNER dulwich.tests.test_suite
 
   # Test with c extensions
   - python setup.py build_ext -i
-  - python -m coverage run -p -m $TEST_RUNNER dulwich.tests.test_suite
+  - python -m coverage run -p --source=dulwich -m $TEST_RUNNER dulwich.tests.test_suite
 after_success:
   - python -m coverage combine
   - codecov