Automated License Check

This patch adds a license checker to the automated tests. This ensures
that only dependencies with licenses from a list of known good licenses
are used.

.licenses.ini

@@ -0,0 +1,9 @@
+# Authorized licenses in lower case
+
+# There is no project rule against adding new licenses as long as they are
+# compatible with the project's license.
+
+[Licenses]
+authorized_licenses:
+    BSD
+    MIT

.travis.yml

@@ -9,12 +9,14 @@ python:
   - "3.7"
 
 install:
-  - pip install flake8 python-coveralls coverage
+  - pip install flake8 python-coveralls coverage liccheck
+  - pip install -r requirements.txt
   - python setup.py bdist_wheel
   - pip install dist/feedgen*
 
 script:
   - make test
+  - liccheck -s .licenses.ini
   - python -m feedgen
   - python -m feedgen atom
   - python -m feedgen rss