build(es5): add es5 checker for build files (#69)

### Summary

Check that non-`esm` files are `ES5`-compatible.

Resolves #57

Author: jaebradley

.travis.yml

@@ -15,6 +15,7 @@ jobs:
         - npm run lint
         - npm run test
         - npm run build
+        - npm run is-build-es5
       before_script: greenkeeper-lockfile-update
       after_script: greenkeeper-lockfile-upload
     - stage: deploy