Removed vendor prefixes, except where it fixes a specific bug or quirk

In reference to issue #558 (https://github.com/Automattic/_s/issues/558).

Some vendor prefixes remain, but only those that explicitly fix a bug or quirk on a device (there are 2 or 3 that fix bugs with the S5).
Webkit vendor prefixes were kept here due to them fixing a bug/quirk in a relatively new browser whose market share is likely growing rather than shrinking.  `box-sizing` is supported in Firefox without prefix since version 29.