Prefer use of a ponyfill for fetch instead of a polyfill

This was causing issues on projects targeting React Native, as the isomorphic-fetch polyfill did not support the correct lookup in the packager when determining what `fetch` module to use.

See JakeChampion/fetch#125 (comment), matthew-andrews/isomorphic-fetch#63

package.json

@@ -53,6 +53,6 @@
   },
   "dependencies": {
     "es6-promise": "^4.1.0",
-    "isomorphic-fetch": "^2.2.1"
+    "fetch-ponyfill": "^4.0.0"
   }
 }

src/index.js

@@ -1,6 +1,6 @@
 /* @flow */
 
-import 'isomorphic-fetch';
+import { fetch } from 'fetch-ponyfill';
 import ClientError from './client_error';
 
 import type {

yarn.lock

@@ -1662,6 +1662,12 @@ fb-watchman@^2.0.0:
   dependencies:
     bser "^2.0.0"
 
+fetch-ponyfill@^4.0.0:
+  version "4.0.0"
+  resolved "https://registry.yarnpkg.com/fetch-ponyfill/-/fetch-ponyfill-4.0.0.tgz#18cfe38d69cde42aec71cb3ace79e1f2276293da"
+  dependencies:
+    node-fetch "~1.6.0"
+
 figures@^1.3.5:
   version "1.7.0"
   resolved "https://registry.yarnpkg.com/figures/-/figures-1.7.0.tgz#cbe1e3affcf1cd44b80cadfed28dc793a9701d2e"
@@ -1979,14 +1985,10 @@ http-signature@~1.1.0:
     jsprim "^1.2.2"
     sshpk "^1.7.0"
 
-iconv-lite@0.4.13:
+iconv-lite@0.4.13, iconv-lite@~0.4.13:
   version "0.4.13"
   resolved "https://registry.yarnpkg.com/iconv-lite/-/iconv-lite-0.4.13.tgz#1f88aba4ab0b1508e8312acc39345f36e992e2f2"
 
-iconv-lite@~0.4.13:
-  version "0.4.15"
-  resolved "https://registry.yarnpkg.com/iconv-lite/-/iconv-lite-0.4.15.tgz#fe265a218ac6a57cfe854927e9d04c19825eddeb"
-
 ignore@^3.2.0:
   version "3.2.4"
   resolved "https://registry.yarnpkg.com/ignore/-/ignore-3.2.4.tgz#4055e03596729a8fabe45a43c100ad5ed815c4e8"
@@ -2201,13 +2203,6 @@ isobject@^2.0.0:
   dependencies:
     isarray "1.0.0"
 
-isomorphic-fetch@^2.2.1:
-  version "2.2.1"
-  resolved "https://registry.yarnpkg.com/isomorphic-fetch/-/isomorphic-fetch-2.2.1.tgz#611ae1acf14f5e81f729507472819fe9733558a9"
-  dependencies:
-    node-fetch "^1.0.1"
-    whatwg-fetch ">=0.10.0"
-
 isstream@~0.1.2:
   version "0.1.2"
   resolved "https://registry.yarnpkg.com/isstream/-/isstream-0.1.2.tgz#47e63f7af55afa6f92e1500e690eb8b8529c099a"
@@ -2760,7 +2755,7 @@ natural-compare@^1.4.0:
   version "1.4.0"
   resolved "https://registry.yarnpkg.com/natural-compare/-/natural-compare-1.4.0.tgz#4abebfeed7541f2c27acfb29bdbbd15c8d5ba4f7"
 
-node-fetch@^1.0.1:
+node-fetch@~1.6.0:
   version "1.6.3"
   resolved "https://registry.yarnpkg.com/node-fetch/-/node-fetch-1.6.3.tgz#dc234edd6489982d58e8f0db4f695029abcd8c04"
   dependencies:
@@ -3665,10 +3660,6 @@ whatwg-encoding@^1.0.1:
   dependencies:
     iconv-lite "0.4.13"
 
-whatwg-fetch@>=0.10.0:
-  version "2.0.3"
-  resolved "https://registry.yarnpkg.com/whatwg-fetch/-/whatwg-fetch-2.0.3.tgz#9c84ec2dcf68187ff00bc64e1274b442176e1c84"
-
 whatwg-url@^4.3.0:
   version "4.5.1"
   resolved "https://registry.yarnpkg.com/whatwg-url/-/whatwg-url-4.5.1.tgz#ba634f630ff0778212c52ea9055d2d061380b1bb"