ci: fail the build if code is not formatted with black

.travis.yml

@@ -24,7 +24,7 @@ install:
 # Python
 - pip freeze
 - pip install -r monkey/monkey_island/requirements.txt  # for unit tests
-- pip install flake8 pytest pytest-cov dlint isort # for next stages
+- pip install black flake8 pytest pytest-cov dlint isort # for next stages
 - pip install coverage  # for code coverage
 - pip install -r monkey/infection_monkey/requirements.txt  # for unit tests
 - pip install pipdeptree
@@ -70,6 +70,9 @@ script:
 ## Check import order
 - python -m isort ./monkey --settings-file ./ci_scripts/isort.cfg
 
+## Check that all python is properly formatted. Fail otherwise.
+- python -m black --check .
+
 ## Run unit tests and generate coverage data
 - cd monkey  # This is our source dir
 - python -m pytest --cov=. # Have to use `python -m pytest` instead of `pytest` to add "{$builddir}/monkey/monkey" to sys.path.