Run an e2e packager test on CircleCI

Summary:Motivation: Catch packager breakages (such as bad shrinkwrap, package.json) quickly and reliably. This is also done as part of the e2e test on Travis. Having an isolated packager test as well makes it easier to see there's a problem with the packager, running in on CircleCI helps detect issues even if the Travis e2e test gets ever broken (have some redundancy, these breakages are pretty painful to fix so we want to detect them as reliably as possible).

**Test plan (required)**

Ran this locally:

    git checkout master
    ./scripts/e2e-test.sh --packager  # The packager created the bundle, exit code was 0
    # Made the packager fail on bad JS
    ./scripts/e2e-test.sh --packager  # Exit code was 1
Closes https://github.com/facebook/react-native/pull/6294

Differential Revision: D3011910

fb-gh-sync-id: 20e8eb29f6aeb3a494a7ef1dbf9cb08fa4e18e27
shipit-source-id: 20e8eb29f6aeb3a494a7ef1dbf9cb08fa4e18e27

circle.yml

@@ -66,6 +66,9 @@ test:
     - ./gradlew :ReactAndroid:connectedAndroidTest -PdisablePreDex --stacktrace --info:
         timeout: 360
 
+    # Publish to Sinopia, create a new app using 'react-native init' and check the packager starts
+    - ./scripts/e2e-test.sh --packager
+
     # testing docs generation is not broken
     - cd website && node ./server/generate.js
   post: