test: add npm artifact build step to Travis

.travis.yml

@@ -30,6 +30,8 @@ before_script:
 script:
   - npm run build
   - ./node_modules/.bin/karma start --single-run --browsers Firefox --reporters mocha
+  # Make sure the build that ships to npm builds without errors
+  - npm run build_npm
   # Can't run until https://github.com/angular/protractor/issues/2784 is resolved
   #- ./node_modules/.bin/protractor protractor.conf.js --browser firefox