release 3.3.5 of Markdown broken. Pin to 3.3.4 to get test to pass.

Python-Markdown/markdown#1196

Author: rouilj

.travis.yml

@@ -101,7 +101,7 @@ install:
   - pip install pytest-cov codecov
   - if [[ $TRAVIS_PYTHON_VERSION != "3.4"* ]]; then pip install docutils; fi
   - if [[ $TRAVIS_PYTHON_VERSION != "3.4"* ]]; then pip install mistune; fi
-  - if [[ $TRAVIS_PYTHON_VERSION != "3.4"* && $TRAVIS_PYTHON_VERSION != "2."* ]]; then pip install Markdown; fi
+  - if [[ $TRAVIS_PYTHON_VERSION != "3.4"* && $TRAVIS_PYTHON_VERSION != "2."* ]]; then pip install Markdown==3.3.4; fi
   - pip install markdown2
   - pip install brotli zstd