Use cross-env to change default port

We should change default port from 3000 to 4100 to prevent conflict
with locally running node/rails backend for Conduit (they also have
default port 3000).

Author: deksden

package.json

@@ -3,6 +3,7 @@
   "version": "0.1.0",
   "private": true,
   "devDependencies": {
+    "cross-env": "^4.0.0",
     "react-scripts": "0.8.5"
   },
   "dependencies": {
@@ -18,9 +19,9 @@
     "superagent-promise": "^1.1.0"
   },
   "scripts": {
-    "start": "react-scripts start",
+    "start": "cross-env PORT=4100 react-scripts start",
     "build": "react-scripts build",
-    "test": "react-scripts test --env=jsdom",
+    "test": "cross-env PORT=4100 react-scripts test --env=jsdom",
     "eject": "react-scripts eject"
   }
 }