Correct HAML version requirement in README

This was relaxed from 4.0.3+ to 4.0+ in fcefd73.

Change-Id: Id80a21745ab80300757320a5b9a35462aaee5b10
Reviewed-on: http://gerrit.causes.com/36633
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Shane da Silva <shane@causes.com>