Update sassy-math to 1.5

The old version of sassy-math (1.2) overrides ruby's `rand()` function, which conflicted with another extension I made: https://github.com/bookcasey/harsh/issues/7

This was patched in 1.5. The function was renamed to `random()`.

Some of your users are still having problems with the old version of sassy-math that is bundled with this app.