ci(Travis): Set max workers to 4 for Jest.

Author: nodkz

.travis.yml

@@ -13,7 +13,7 @@ node_js:
   - "5"
   - "4"
 script:
-  - yarn run coverage
+  - yarn run coverage -- --maxWorkers 4
   - yarn run build
 after_success:
   - 'curl -Lo travis_after_all.py https://git.io/travis_after_all'