Disable 3D CSS transforms on PhantomJS

PhantomJS does not support 3D CSS transform even though it's a Webkit product. This patch detects PhantomJS in the user agent and disables 3D transforms.

Details on PhantomJS support are here: https://groups.google.com/forum/#!msg/phantomjs/h7-4-CWsy1g/KX1tlf5gT5IJ
Merge pull request #1501 from rassie/patch-1

Disable 3D CSS transforms on PhantomJS