Including stylesheet leads to warning in latest chrome

Including the latest bootstrap chosen stylesheet in latest chrome, leads to the following warning in developer console:

```text
Consider using 'dppx' units, as in CSS 'dpi' means dots-per-CSS-inch, not dots-per-physical-inch, 
so does not correspond to the actual 'dpi' of a screen. In media query expression: only screen and 
(-webkit-min-device-pixel-ratio: 2), only screen and (min-resolution: 144dpi) 
```