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