Force touch support off for PhantomJS

PhantomJS has `'ontouchstart' in document.documentElement`, but
doesn't actually support touch.

http://code.google.com/p/phantomjs/issues/detail?id=375
https://github.com/ariya/phantomjs/pull/408
https://github.com/Leaflet/Leaflet/pull/1434#issuecomment-13843151