Pin mysqlclient version for testing

mysqlclient 1.4.0 has a problem with Django below version 2. See PyMySQL/mysqlclient#306

Author: Alex Hill

.travis.yml

@@ -47,7 +47,7 @@ python:
   - "3.6"
 # Tell it the things it will need to install when it boots
 install:
-  - pip install -q pytz coveralls flake8 psycopg2 mysqlclient
+  - pip install -q pytz coveralls flake8 psycopg2 mysqlclient==1.3.14
   - pip install -q "Django==$DJANGO_VERSION"
   - pip install -e .
 # Tell Travis how to run the test script itself