allow results columns down to bootstrap  size

* Will match the breakpoint for facets collapsing behind a 'show' icon
* For expected typical content, seems to work fine at this size,
  with better functionality than collapsing to single-column
* will still collapse to single column below 'sm' size, the
  same point that facets collapse to be hidden behind a 'show' icon
Merge pull request #924 from projectblacklight/smaller_results_columns

allow results columns down to bootstrap  size