Don't publish /ReactAndroid/build to npm, update version on master

Summary: @​public

See the discussion in https://github.com/facebook/react-native/pull/3019

This is temporary Gradle output and can be huge (>100MB).

Reviewed By: @foghina

Differential Revision: D2531612

fb-gh-sync-id: 8874d39b1a9b35dc4b4ce465dd149589db75bb29

ReactAndroid/.npmignore

@@ -0,0 +1,3 @@
+# Make sure we never publish ReactAndroid/build (Gradle output) to npm.
+# The folder is huge (> 100MB)!
+build/

package.json

@@ -1,6 +1,6 @@
 {
   "name": "react-native",
-  "version": "0.8.0",
+  "version": "0.12.0",
   "description": "A framework for building native apps using React",
   "license": "BSD-3-Clause",
   "repository": {