Add travis configuration file.

At some point, I'd like to get the pre-commit tests up and
running on travis.  But those will be much more difficult
to set up -- they require a good deal of fixtures that
I don't want to add to the repository.

Resolves: Issue #72.

.travis.yml

@@ -0,0 +1,18 @@
+language: python
+
+dist: xenial
+
+install: pip install tox pytest
+
+script: tox
+
+matrix:
+  include:
+    - env: TOXENV=py35
+      python: 3.5
+    - env: TOXENV=py36
+      python: 3.6
+    - env: TOXENV=py37
+      python: 3.7
+    - env: TOXENV=py38
+      python: 3.8

tox.ini

@@ -1,13 +1,13 @@
 # content of: tox.ini , put in same dir as setup.py
 [tox]
-envlist = py35,py38
+envlist = py35,py36,py37,py38
 [pytest]
 addopts = --ignore bin --ignore integration_tests/repos
 
 [testenv]
 deps=pytest
 commands=
-  pytest --ignore=integration_tests/
+  pytest --ignore=integration_tests/ --ignore=_data/
 
 # TODO: Add flake8 output.
 [testenv:pre-commit]