Remove -moz-box-sizing

[mgol]

Unprefixed box-sizing has been available since Firefox 29 which went out a year & 3 months ago; market share of Firefox < 29 is nearly non-existent.

I don't think it's worth keeping the -moz-box-sizing version.

[marcobiedermann]

@mzgol Take a look at #400

[mgol]

I should have looked at current master before reporting. :)