Make it compatible with Python 3.7+

[Mariatta]

We observed the following deprecation warning in Python 3.7.1+:

Travis CI log
PR: python/bedevere#154
Issue: python/bedevere#153

../../../virtualenv/python3.8-dev/lib/python3.8/site-packages/uritemplate/variable.py:363: in dict_test
    return isinstance(value, (dict, collections.MutableMapping))

DeprecationWarning: Using or importing the ABCs from 'collections' instead of from 'collections.abc' is deprecated, and in 3.8 it will stop working

In this PR I updated the import to avoid the DeprecationWarning and add Python 3.7 to travis CI matrix.

Thanks for this useful library!

[sigmavirus24]

This was already fixed in #40 but hasn't been released yet (it would seem)

[Mariatta]

Thanks for the info! I confirmed that if we were to use the latest version, it will work.
I've updated my PR just to make the tests running under Python 3.7+ environments. Thanks again.

[hugovk]

Looks like this has been superseded by #45.