Pin black to specific version (#609)

Author: Ross Mechanic

.travis.yml

@@ -21,8 +21,8 @@ install:
   - pip install -U coverage codecov
   - pip install -U flake8==3.7.9
   - pip install -U $DJANGO
-  - if [[ $TRAVIS_PYTHON_VERSION == '3.6' ]]; then pip install black; fi
-  - if [[ $TRAVIS_PYTHON_VERSION == '3.7' ]]; then pip install black; fi
+  - if [[ $TRAVIS_PYTHON_VERSION == '3.6' ]]; then pip install black==19.10b0; fi
+  - if [[ $TRAVIS_PYTHON_VERSION == '3.7' ]]; then pip install black==19.10b0; fi
   - pip freeze
 
 script: