Check for modified files in Travis

Author: seanf

.travis.yml

@@ -3,4 +3,10 @@ language: node_js
 node_js:
   - "6"
 script:
-  - "yarn build-ci"
+  - yarn build-ci
+  # Ideally we wouldn't commit transpiled .js or .js.map files, but for now:
+  - if ! git diff --exit-code --name-only HEAD
+    then
+      echo "Changed files detected (listed above). Please run 'yarn build' before you commit."
+      exit 1
+    fi