chore: downgrade mocha

Mocha v5 does not support browsers <IE11

Author: citizensas

package.json

@@ -40,7 +40,7 @@
     "karma-sauce-launcher": "1.2.0",
     "karma-sourcemap-loader": "0.3.7",
     "karma-webpack": "2.0.9",
-    "mocha": "5.0.0",
+    "mocha": "4.1.0",
     "phantomjs-prebuilt": "2.1.16",
     "semver": "5.5.0",
     "should": "13.1.1",