Add missing dev dep

Author: wadetandy

package.json

@@ -48,6 +48,7 @@
     "@types/winston": "2.3.7",
     "chai": "^4.1.2",
     "chai-things": "^0.2.0",
+    "es6-promise": "4.2.4",
     "fetch-mock": "^5.10.0",
     "husky": "0.14.3",
     "isomorphic-fetch": "^2.2.1",