Note that transition cache is currently available only on master

@nicka reported via https://github.com/rails/turbolinks/issues/327 that he tried using `Turbolinks.enableTransitionCache` but got a no method error. This is because he was running 2.1.0, which does not include transition cache.

I added this note to make it clear to point the turbolinks gem to master. Maybe it's worthwhile to bump to 2.1.1 to make transition cache a bit easier to use?