Have Black check also show the diff

If Black detecs the need to format a file it will show the relevant diff

.travis.yml

@@ -14,7 +14,7 @@ install:
 script:
   - pyflakes pyhf
   - pytest -r sx --ignore tests/benchmarks/ --ignore tests/test_notebooks.py
-  - if [[ $TRAVIS_PYTHON_VERSION == '3.6' ]]; then black --check --verbose .; fi
+  - if [[ $TRAVIS_PYTHON_VERSION == '3.6' ]]; then black --check --diff --verbose .; fi
 after_success: coveralls
 
 # always test (on both 'push' and 'pr' builds in Travis)