Add python 3.6 for black

Author: chdemko

.travis.yml

@@ -5,6 +5,7 @@ before_install:
   - pandoc -v
   - sudo apt-get install python2.7
   - sudo apt-get install python
+  - sudo apt-get install python3.6
   - sudo apt-get install python-pip
   - pip install --user --upgrade pip
 install: