Fix appcache usage of WebApp.clientHash

In https://github.com/meteor/meteor/commit/71966f407a651643424ba83d854c658e6a957542#diff-79c5e18978e1bfa64cbb9edb380f2920L214
`WebApp.clientHash` changed from a string to a
function.

This PR fixes the appcache package to call the new function.

With the change to a function being called, the client hash is no
longer cached, and the client hash will be recalculated on each load of
the app manifest file.

I presume the client hash could be cached by the appcache package
because it always uses the default architecture ("web.browser"), but I
didn't make that addition because I wasn't sure if that was correct.