Fix DefaultReporter class constant reference

We were getting a report of this breaking on Windows machines. While I
don't have a Windows box handy, my hunch is that it has something to do
with the `SCSSLint` namespace prefix missing from the constant
reference.

Change-Id: I9763173442f23ff7f6e2b1b4578811f89f80f05a
Reviewed-on: http://gerrit.causes.com/38008
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Shane da Silva <shane@causes.com>