Included respond.js which comes with bootstrap

This was missing from _strap causing IE8 to force mobile views on desktop.
let's not duplicate the ie conditional `<!--[if lt IE 9]>` (opened a [few lines above](https://github.com/ptbello/_strap/pull/6/files#diff-1c1ae65fe9828eb6d3cee4edacf90018R16), let's use that one)