Sass 3.3 compatibility
Noticed it by accident when checking out the Travis builds  https://travis-ci.org/twbs/ratchet/builds/20333914

Example:

"DEPRECATION WARNING on line 17 of sass/theme-ios.scss:
Assigning to global variable "$chrome-color" by default is deprecated.
In future versions of Sass, this will create a new local variable.
If you want to assign to the global variable, use "$chrome-color: rgba(247, 247, 247, 0.98) !global" instead.

Note that this will be incompatible with Sass 3.2."
