Update _rem.scss

The rem value output generated an extra space between the value and rem. By moving 'rem' to the output itself, this issue was fixed.

I don't know if more people had this issue, but I got this output by using the previous version:

font-size: 40px;
font-size: 2.5 rem;

With my change it outputs correctly:

font-size: 40px;
font-size: 2.5rem;

Hope this helps more people...