fixed travis configuration to use full test set.

Now when branches are pushed or PRs are opened, travis will run py37full
instead of just py37. Running `tox` locally will still use py37 for ease
of development. py37full can be run with `tox -e py37full` but does not
run by default because it contains tests that can be time consuming.

.travis.yml

@@ -15,7 +15,7 @@ jobs:
       env:
         - TOXENV=docs
     - env:
-        - TOXENV=py37,codecov
+        - TOXENV=py37full,codecov
       python: '3.7'
 before_install:
   - python --version