Change detection of BSD vs GNU date

Previous detection was checking for uname Linux/Debian,
but Debian can run in a GNU environment as well.

Ref: http://stackoverflow.com/questions/8747845/how-can-i-detect-bsd-vs-gnu-version-of-date-in-shell-script
Merge pull request #37 from andreineculau/patch-2

Change detection of BSD vs GNU date