Change isomorphic-fetch to fetch-ponyfill

package.json

@@ -45,7 +45,7 @@
     "debug": "^2.6.8",
     "detect-node": "^2.0.3",
     "ecurve": "^1.0.5",
-    "isomorphic-fetch": "busyorg/isomorphic-fetch",
+    "fetch-ponyfill": "^4.1.0",
     "lodash": "^4.16.4",
     "postinstall-build": "^5.0.1",
     "secure-random": "^1.1.1",

src/api/transports/http.js

-import fetch from 'isomorphic-fetch';
+import fetchPonyfill from 'fetch-ponyfill';
+import Promise from 'bluebird';
 import newDebug from 'debug';
-
 import Transport from './base';
 
+const { fetch } = fetchPonyfill(Promise);
 const debug = newDebug('steem:http');
 
 export default class HttpTransport extends Transport {