Remove .babelrc from the published package

Installed from npm, this package doesn't work with React Native,
because although the released code is transpiled, .baberc is
also published to npm.

Fixed by adding the `files` property in package.json to prevent .babelrc from being published.

See:
facebook/react-native#4062

package.json

@@ -21,6 +21,10 @@
     "type": "git",
     "url": "https://github.com/ForbesLindesay/redux-optimist.git"
   },
+  "files": [
+    "index.js",
+    "lib"
+  ],
   "author": "ForbesLindesay",
   "license": "MIT"
-}
+}