Change ColorKeyword to use source_from_range again

This was changed in f3a8839, but actually changes the behavior as the
Sass parser can potentially normalize the values here. We want to ensure
we are _always_ reading from the original source.

Change-Id: Ibeeb7525ff943018821312e2708db978d465e384
Reviewed-on: http://gerrit.causes.com/37974
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Shane da Silva <shane@causes.com>