If install of zstd fails, don't abort the test run.

zstd is optional. If it fails allow the test run to continue.

Author: rouilj

.travis.yml

@@ -108,7 +108,10 @@ install:
   - if [[ $TRAVIS_PYTHON_VERSION != "3.4"* ]]; then pip install mistune==0.8.4; fi
   - if [[ $TRAVIS_PYTHON_VERSION != "3.4"* && $TRAVIS_PYTHON_VERSION != "2."* ]]; then pip install Markdown; fi
   - pip install markdown2
-  - pip install brotli zstd
+  - pip install brotli
+  # zstd fails to build under python nightly aborting test.
+  # allow testing to still happen if the optional package doesn't install.
+  - pip install zstd || exit 0
 
 before_script:
   # set up mysql database