Warning: Toggles.checkbox will be briefly broken in Firefox 49/50

[dannyob]

It's not elm-mdl's fault, but a regression in the Firefox 49 beta and Firefox 50 nightly builds means that the Material Design checkbox rendering displays incorrectly in those browsers -- the checkbox will display, but the checkmark will not appear. @debois suggested I leave a note here just to save any temporary confusion.

Relevant bug is here: https://bugzilla.mozilla.org/show_bug.cgi?id=1294171

The fix is already checked in, and should be shipped to the browsers in the next week or so.

[dannyob]

Fix is now available from the Aurora/nightly branch, and should appear in Beta shortly. I'm going to mark this as fixed.