Fix travis build setup

Author: Nabellaleen

.travis.yml

@@ -9,7 +9,7 @@ before_install:
 install:
 - |
   if [ "$TEST_TYPE" = build ]; then
-    pip install pytest>=3.6 pytest-cov pytest-benchmark coveralls six mock sqlalchemy_utils
+    pip install "pytest>=3.6" pytest-cov pytest-benchmark coveralls six mock sqlalchemy_utils
     pip install -e .
     python setup.py develop
   elif [ "$TEST_TYPE" = lint ]; then