Use bootstrap breakpoints by default

Since bootstrap is the de facto default styling of the web now, it makes
sense to use those breakpoints by default.

Anecdotally, I quite enjoy the syntax that sass-enhance provides, and
so I like bringing it in to personal projects. But most of those
personal projects use Bootstrap, so I end up configuring sass-enhance
more than necessary.

Change-Id: I4a215ab612b939a955394d0609914f91b8a10b0c