travis: use mysql client in docker container for polling

Author: julienschmidt

.travis/wait_mysql.sh

@@ -1,7 +1,7 @@
 #!/bin/sh
 while :
 do
-    if mysql -e 'select version()' 2>&1 | grep 'version()\|ERROR 2059 (HY000):'; then
+    if docker exec mysqld mysql -e 'select version()'; then
         break
     fi
     sleep 3