Tests: Try to run tests using 4 workers on Travis

package.json

@@ -139,7 +139,7 @@
     "build-storybook": "build-storybook",
     "test-unit": "jest",
     "test-unit:coverage": "jest --coverage",
-    "test-unit:coverage-ci": "jest --coverage -i && coveralls < coverage/lcov.info",
+    "test-unit:coverage-ci": "jest --coverage -w=4 && coveralls < coverage/lcov.info",
     "test-unit:watch": "jest --watch"
   }
 }