Python3.7, testing and bug fixes

* Upgrade to python3.7 and pandas 0.3.4 because pandas has dropped support for
python 3.4 -> There are some API changes in pandas, and I've update the code
accordingly.
* Set pytest as the default test runner

examples/pubcrawl/pubcrawl.py

@@ -4,6 +4,8 @@ from random import random, shuffle
 from itertools import islice
 import logging
 
+import other_module
+
 
 class CityPubs(Environment):
     '''Environment with Pubs'''