Fix debian install due to mariadb-client-core circular conflict

Aptitude provided the following error message without this fix:
 mariadb-client-core-10.3 :
    Conflicts: mysql-client-5.1 which is a virtual package, provided by:
             - mariadb-client-10.3, but 10.3.0+maria~xenial is to be installed.
    Conflicts: mysql-client-5.5 which is a virtual package, provided by:
             - mariadb-client-10.3, but 10.3.0+maria~xenial is to be installed.

debian/control

@@ -194,8 +194,8 @@ Conflicts: mariadb-client-10.0,
            mariadb-client-core-5.5,
            mysql-client (<< 5.0.51),
            mysql-client-5.0,
-           mysql-client-5.1,
-           mysql-client-5.5,
+           mysql-client-5.1 (<< ${source:Version}),
+           mysql-client-5.5 (<< ${source:Version}),
            mysql-client-core-5.1,
            mysql-client-core-5.5,
            mysql-client-core-5.6,