Fix for error in Firefox because of workaround for Safari

Fix for https://github.com/Polymer/platform/issues/94
RegExp based on user agent examples from http://myip.ms/view/comp_browsers/3458/Safari_8.html beause I do not have any available machine with Safari 8
Yes, it may work. And will be an elegant fix.
But I'm unable to check it in real Safari 8.
Just checked in Firefox Nightly - your variant works fine.