fix for chrome on iOS 6.1.3

In Chrome on iOS 6.1.3, calling Turbolinks.visit will fail because
browserSupportsPushState returns true, but currentState is still null
(because browserIsntBuggy returns false) and thus the cacheCurrentPage
function will fail. This one-line fix ensures that when
Turbolinks.visit is called, if either browserSupportsPushState or
browserIsntBuggy returns false, we fall back to simply setting
document.location.href.