Fix pytest-cov installation errors

Author: alanjds

.travis.yml

@@ -41,7 +41,7 @@ before_script:
   # https://github.com/travis-ci/travis-ci/issues/8920#issuecomment-352661024
   - python -c "import fcntl; fcntl.fcntl(1, fcntl.F_SETFL, 0)"
   - pip install --upgrade pip
-  - pip install pytest pytest-cov coverage
+  - pip install 'pytest==3.8.1' 'pytest-cov==2.6.0'
 
 script:
   # Install the thing