moved promise polyfill to dev dependencies

Author: zashraf1985

packages/optimizely-sdk/package.json

@@ -40,7 +40,6 @@
     "json-schema": "^0.2.3",
     "lodash": "^4.17.11",
     "murmurhash": "0.0.2",
-    "promise-polyfill": "8.1.0",
     "uuid": "^3.3.2"
   },
   "devDependencies": {
@@ -62,7 +61,8 @@
     "nock": "^7.7.2",
     "sinon": "^2.3.1",
     "webpack": "^4.25.1",
-    "webpack-cli": "^3.1.2"
+    "webpack-cli": "^3.1.2",
+    "promise-polyfill": "8.1.0"
   },
   "publishConfig": {
     "access": "public"