Travis CI: Add SherlockSiteCoverageTests as a parallel run

As suggested at https://github.com/sherlock-project/sherlock/issues/235#issuecomment-511245466

Should we remove the __|| true__ from the end of line 15 or line 24?  Or leave them as is?

.travis.yml

@@ -9,6 +9,10 @@ matrix:
     - python: 3.6
     - python: 3.7
     - python: nightly
+    - python: 3.7
+      before_script: true  # override the flake8 tests
+      name: "Sherlock Site Coverage Tests"
+      script: python -m unittest tests.all.SherlockSiteCoverageTests --buffer --verbose || true
 install:
   - pip install flake8 -r requirements.txt
 before_script: