Add dppx to Retina media query

This addresses #1771, which is now fixable because the Chrome team has 
fixed the erroneous "Consider using dppx" console message.

See also:
https://code.google.com/p/chromium/issues/detail?id=336276
http://css-tricks.com/forums/topic/chrome-console-recommends-dppx-instead-of-dpi-even-if-dppx-is-used/#post-187595