chore(release): don't update phonecat and seed during a release

This is no more needed as phonecat and seed are using bower
now to get the angular version.
chore(release): calculate the cdnVersion on every build

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.
chore(release): remove after CDN script

The homepage (angularjs.org) and the docs now calculate the
current cdn version on every build, so there is no need
for an after-cdn script.
chore(release): simplify scripts so that they can be tested locally

The `git fetch --all` resulted in an error if in the local `.gitconfig`
a remote was configured that does not exist in the bower/code.anguarjs.org
repositories (e.g. "remote "upstream-prs"").