Remove caret from "react": "^16.0.0-alpha.12"

`npm install` / `yarn install` auto updates to `16.0.0-alpha.13` which
breaks the proptypes import.

Removing caret to prevent auto update to `alpha.13`.

For more information:

facebook/react-native#14454

Author: sagi

package.json

@@ -50,7 +50,7 @@
     "gitbook-cli": "^2.3.0",
     "lodash": "^4.17.2",
     "prop-types": "^15.5.10",
-    "react": "^16.0.0-alpha.12",
+    "react": "16.0.0-alpha.12",
     "react-native": "^0.45.1"
   },
   "rnpm": {