Update _pxToEm.scss

Added a check for if the value 'auto' is passed to the function to deal with cases where auto is desired as an output.  for example:

margin: pxToEm(20px auto, 16px); would output -> margin: 1.25em auto;
Merge pull request #163 from MrFjords/patch-1

Update _pxToEm.scss