Uses Adaptive.$ over Mobify.$ if it is defined

Uses Adaptive.$ over Mobify.$ if it is defined

Status: **Opened for visibility**
Reviewers: **@scalvert @jansepar @donnielrt @kpeatt **

## Changes
- binds $ to Adaptive.$ if it's defined, otherwise falls back to Mobify.$ as before

## Notes
- this change is so we can continue to use bellows as a bower component beyond adaptive.js 0.1.18, since we extend Adaptive's selectorEngine with bellows
- backwards-compatible