Formats all .scala files according to `scalafmt`'s (opinionated) style. 
Adds Travis checks for correctly formatted code.

Update tests to work in both Python 2 and Python 3.
Rename pythonaction to python3action for container image name for clarity.
Add tests for python:2 and python:3.
Add image names for all the actions. Rename javaaction to java8action for consistency.