chore(package.json): update docs app to use version 1.8 of AngularJS

As mentioned in `RELEASE.md`, now that the [CDN][1] has been updated
with the 1.8.0 version, it is safe to bump the value of the
`branchVersion` property in `package.json` to `^1.8.0`. This will cause
the docs app to use the latest version, namely 1.8.0.

[1]: https://ajax.googleapis.com/ajax/libs/angularjs/1.8.0/angular.js

package.json

@@ -1,7 +1,7 @@
 {
   "name": "angular",
   "license": "MIT",
-  "branchVersion": "^1.7.0",
+  "branchVersion": "^1.8.0",
   "branchPattern": "1.8.*",
   "distTag": "next",
   "repository": {