update superagent to the latest

Summary:
all tests passing (255 tests)

update superagent to 1.4.0 (latest)

react-native requires a newer version of superagent
that at least includes a PR from react-native team:
ladjs/superagent#621

Reviewers: ben, benchan

Reviewed By: benchan

Subscribers: ben

Differential Revision: https://phabricator.bitgo.com/D1778

package.json

@@ -69,8 +69,8 @@
     "minimist": "0.2.0",
     "morgan": "1.5.3",
     "q": "1.1.2",
-    "superagent": "0.18.0",
-    "superagent-proxy": "0.3.1",
+    "superagent": "^1.4.0",
+    "superagent-proxy": "^1.0.0",
     "underscore.string": "2.4.0"
   }
 }