Remove @import of Compass CSS3 mixins

Added erroneously in 10fc285f.

Mixin naming collisions meant vendor-prefixed flex box properties were
being generated inconsistently, breaking our layout in IE 10.

With this commit, we now rely on only one CSS mixin library: Bourbon.

Signed-off-by: Alex Coles <alex@alexbcoles.com>
Revert "Tell Travis to test against Firefox 24"

Custom versions of Firefox are not available when `sudo: false` is
specified. See d5969048.

This reverts commit 8ab66fcbc5c0c57e0a5871dfe18adc1d2cf02e15.