ci(circle config): add a circle CI config

This only runs the JS build (no tests) as an easy place to start.

Green build on my branch: https://circleci.com/gh/alexeagle/angular/5

Note, we are just experimenting with Circle at this point...

Closes #6520

circle.yml

@@ -0,0 +1,6 @@
+machine:
+  node:
+    version: 4.2.1
+test:
+  override:
+    - npm run build

package.json

@@ -28,6 +28,7 @@
   "scripts": {
     "preinstall": "node tools/analytics/build-analytics start install npm-install && node tools/analytics/build-analytics start install npm-preinstall && node tools/npm/check-node-modules --purge && node tools/analytics/build-analytics success install npm-preinstall && node tools/analytics/build-analytics start install npm-install-net",
     "postinstall": "node tools/analytics/build-analytics success install npm-install-net && node tools/analytics/build-analytics start install npm-postinstall && node tools/npm/copy-npm-shrinkwrap && node tools/chromedriverpatch.js && webdriver-manager update && bower install && gulp pubget.dart && tsd reinstall --overwrite --clean --config modules/angular2/tsd.json && tsd reinstall --overwrite --clean --config tools/tsd.json && tsd reinstall --overwrite --config modules/angular1_router/tsd.json && node tools/analytics/build-analytics success install npm-postinstall && node tools/analytics/build-analytics success install npm-install",
+    "build": "gulp build.js",
     "test": "gulp test.all.js && gulp test.all.dart"
   },
   "dependencies": {