Move git diff --exit-code so build fails instead of errors

MSP-11616

With `git diff --exit-code` in `before_script` it caused the build to
show as errored in travis and didn't turn the merge button red, so try
moving to the `script` section to see if that will make the merge button
red.

.travis.yml

@@ -20,9 +20,10 @@ before_script:
   - bundle exec rake --version
   - bundle exec rake db:create
   - bundle exec rake db:migrate
+script:
   # fail build if db/schema.rb update is not committed
   - git diff --exit-code
-script: "bundle exec rake $RAKE_TASK"
+  - "bundle exec rake $RAKE_TASK"
 
 rvm:
   - '1.9.3'