Use ellipses to truncate long constraint values at responsive breakpoints

Fixes #925, maybe, @jronallo?

@jkeck, I saw we were truncating values in searchworks. Do you think these values are close enough to searchworks, or could they use some tweaking?

For reference:

```css

// Extra small screen / phone
$screen-xs:                  480px !default;
$screen-sm:                  768px !default;
$screen-md:                  992px !default;
$screen-lg:                  1200px !default;
```

![screen shot 2014-11-10 at 2 34 23 pm](https://cloud.githubusercontent.com/assets/111218/4985127/08a22610-692a-11e4-8201-60e30612d1a4.png)
![screen shot 2014-11-10 at 2 34 40 pm](https://cloud.githubusercontent.com/assets/111218/4985128/08ba5726-692a-11e4-8a25-38da03a58ac7.png)
![screen shot 2014-11-10 at 2 34 28 pm](https://cloud.githubusercontent.com/assets/111218/4985130/08c174e8-692a-11e4-97c9-c8f15f247c89.png)
![screen shot 2014-11-10 at 2 34 35 pm](https://cloud.githubusercontent.com/assets/111218/4985129/08c07c64-692a-11e4-8a23-651fa2399c64.png)