Don't ignore touch events on iOS 7.1.2 or later.

As a partial resolution for #17, I'm proposing that iOS touch events not be ignored for versions of iOS known to not exhibit the bug where a bogus event is generated after alert()/prompt() calls.  Unfortunately, I can only test down to iOS 7.1.2, but until I get an iOS 6 device this will at least fix #17 on some devices (without breaking it on others).