Travis CI and tox.ini: Add Python 3.7 to the testing

[cclauss]

One warning to be aware of for Python 3.8...

=============================== warnings summary ===============================
tests/main_test.py::TestMain::test_reads_from_stdin
  /home/travis/build/Yelp/detect-secrets/.tox/py37/lib/python3.7/site-packages/yaml/constructor.py:126:
  DeprecationWarning: Using or importing the ABCs from 'collections' instead of from 'collections.abc' is deprecated, and in 3.8 it will stop working
    if not isinstance(key, collections.Hashable):

[KevinHock]

@cclauss strikes again :) Your PyT PRs were great. Thanks for making this.

[KevinHock]

LGTM, merging now :)

[joshuarli]

@KevinHock @cclauss Xenial is now generally available on Travis, you should be able to have a top-level dist: xenial declaration instead of putting it in the matrix.

[cclauss]

I can confirm that both solutions work. While xenial is generally available, it is not the Travis default so I usually take the more conservative route of only using it where it is needed.

[joshuarli]

Good point, it's nice to be explicit about nondefaults.