Add documentation for dummy variables in .travis.yml (#5781)

.travis.yml

@@ -21,6 +21,7 @@ env:
     - RAILS_ENV=test
     - CC_TEST_REPORTER_ID=f39e060a8b1a558ebd8aff75d5b9760bf1ae98f3f85d628ae28814f3c66438cd
     - DATABASE_URL=postgres://postgres@localhost/
+    # Dummy values needed to verify the app boots via "rails runner"
     - APP_PROTOCOL=http://
     - APP_DOMAIN=localhost:3000
     - TIMBER=dummy