Downgrade to Protractor v4

v5 requires Node.js v6.9.0 and higher.

package.json

@@ -67,8 +67,8 @@
     "istanbul-coveralls": "^1.0.3",
     "mkdirp": "^0.5.1",
     "mocha": "2.5.2",
-    "protractor": "5.0.0",
-    "protractor-cucumber-framework": "0.6.0",
+    "protractor": "^4.0.14",
+    "protractor-cucumber-framework": "^0.6.0",
     "sinon": "1.17.4",
     "sinon-chai": "2.8.0",
     "supertest": "1.2.0",