Remove breakpoints sort

Sorting the breakpoints yields bad results if not all sizes are of the same length.
For instance, for the breakpoints [ '480px', '768px', '992px', '1200px', '1800px', '2200px' ], the first 3 sizes will be placed after the last 3, resulting in unexpected behaviour.
Merge pull request #759 from yelly/master

Remove breakpoints sort