Fix running the tests with Node.js < 8, which did not run npm prepare

Author: prantlf

.travis.yml

@@ -10,6 +10,9 @@ node_js:
   - '8'
   - '6'
   - '4'
+scripts:
+  - npm run build
+  - npm test
 after_success:
   - npm run coverage
   - npx semantic-release