Produce coverage report only on stable Perl version

It's only really necessary to produce the coverage report the once and
it makes most sense to do this on a non-blead perl.

Author: paultcochrane

.travis.yml

@@ -5,7 +5,7 @@ matrix:
         - perl: "5.24"
           env: COVERAGE=1 MYSQL_VERSION=5.7
         - perl: "blead"
-          env: COVERAGE=1 MYSQL_VERSION=5.7
+          env: MYSQL_VERSION=5.7
     allow_failures:
         - perl: "blead"
           env: MYSQL_VERSION=5.7