Fixed build of Docker container on Travis

.travis.sh

@@ -40,6 +40,10 @@ echo "$TRAVIS_BRANCH" | grep -qvE "^[0-9.]+$" && exit 0; :;
 # Install dependencies for release build
 pip install --user wheel twine
 
+# Fix SSL warnings for Python > 2.7.9
+# https://urllib3.readthedocs.io/en/latest/user-guide.html#ssl-py2
+pip install --user urllib3[secure]
+
 # Build and install theme and Docker image
 python setup.py build sdist bdist_wheel --universal
 docker build -t $TRAVIS_REPO_SLUG .