The CDN version of angular is now calculated on every build,
by looking at the tags in angular/angular.js, sorting them
by semver and checking against ajax.googleapis.com which
one is available.
`git ls-remote --tags` assumes that you have a remote set up for your
current branch. That isn't the case, at least for me, when I'm working
on local branches. `grunt write` doesn't do the right thing in that
case (`git ls-remote --tags` bails out and the silent: true param makes
this a pain to debug.) Prefer explicit to implicit.
Closes#6678.
The docs were relying on the grunt/util module for getting version info
but this was unreliable and full of custom regexes. This is moved into
a new version-info module that makes much better use of the semver library.
