Get latest release from dist-tags

Rather than getting the stable release from time, it should be grabbed from `dist-tags` so that update-notifier doesn't incorrectly tell users to upgrade to prereleases. This should solve that problem.

Tests passing, but the testing of this particular feature would be more valid if you pushed a prerelease of 'generator-backbone', which seems to be the object used in the tests, or perhaps if another npm project was set up just for testing this thing which was never really updated and had a prerelease pushed.