Clearbricks: Only cope with utf8mb4 wit 5.7+ MySQL server in order to cope with long indexes

"innodb_large_prefix is enabled by default in MySQL 5.7. This change coincides with the default value change for innodb_file_format, which is set to Barracuda by default in MySQL 5.7. Together, these default value changes allow larger index key prefixes to be created when using DYNAMIC or COMPRESSED row format. If either option is set to a non-default value, index key prefixes larger than 767 bytes are silently truncated." ( https://dev.mysql.com/doc/refman/5.7/en/innodb-parameters.html#sysvar_innodb_large_prefix)