fix(tests): work around Travis port collision

.travis.yml

@@ -24,6 +24,7 @@ env:
    - DESECSTACK_DB_SUBJECT_ns2replication=9Fn33T5yGukjnrtj
    - DESECSTACK_NSLORD_APIKEY=9Fn33T5yGukjekwjew
    - DESECSTACK_NSMASTER_APIKEY=LLq1orOQuXCINUz4TV
+   - DESECSTACK_DBMASTER_PORT=13306
    - DESECSTACK_IPV4_REAR_PREFIX16=172.16
    - DESECSTACK_IPV6_SUBNET=bade:affe:dead:beef:b011::/80
    - DESECSTACK_IPV6_ADDRESS=bade:affe:dead:beef:b011:0642:ac10:0080

docker-compose.yml

@@ -75,7 +75,7 @@ services:
     build: dbmaster
     image: desec/dedyn-dbmaster:latest
     ports:
-    - "3306:3306"
+    - "${DESECSTACK_DBMASTER_PORT:-3306}:3306"
     volumes:
     - ${DESECSTACK_DBMASTER_CERTS}:/etc/ssl/private:ro
     - dbmaster_mysql:/var/lib/mysql