Update .travis.yml (#1557)
Update .travis.yml (#1557)

Add support for Python 3.7. Fix #1552
smalley/python,exercism/xpython,exercism/python,jmluy/xpython,behrtam/xpython,behrtam/xpython,exercism/python,jmluy/xpython,exercism/xpython,smalley/python,N-Parsons/exercism-python,N-Parsons/exercism-python