[react-packager] Switch from Q to Bluebird as promises library

Summary:
This PR improves performance of `react-packager` by switching the promises library from the [Q](https://github.com/kriskowal/q) to [Bluebird](https://github.com/petkaantonov/bluebird).

[Here is the test result](https://github.com/facebook/react-native/issues/361#issuecomment-87829808) showing a noticeable difference. (2x speed improvement)

Please refer to [this issue](https://github.com/facebook/react-native/issues/361) for more details.
Closes https://github.com/facebook/react-native/pull/516
Github Author: Pilwon Huh <pilwon@gmail.com>

Test Plan:
./runJestTests
start app and click around

package.json

@@ -53,13 +53,13 @@
     "joi": "~5.1.0",
     "module-deps": "3.5.6",
     "optimist": "0.6.1",
-    "q": "1.0.1",
     "sane": "1.0.1",
     "uglify-js": "~2.4.16",
     "underscore": "1.7.0",
     "worker-farm": "1.1.0",
     "yargs": "1.3.2",
-    "ws": "0.4.31"
+    "ws": "0.4.31",
+    "bluebird": "^2.9.21"
   },
   "devDependencies": {
     "jest-cli": "0.2.1",