Shorter version column, use default charset for mysql

[amacneil]

• Use default charset for mysql
• Change version column from varchar(256) to varchar(128) to avoid issues with utf8mb4 on mysql. Made this change on all drivers for consistency.

Refs:

• Support custom charset and collation for schema_migrations table (mysql) #390
• On MySQL 5.6 with default utf8mb4 encoding, schema_migrations table fails to create #85

[amacneil]

@mheffner can you test

[mheffner]

Looks good!

> show create table schema_migrations;
+-------------------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Table             | Create Table                                                                                                                                                       |
+-------------------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| schema_migrations | CREATE TABLE `schema_migrations` (
  `version` varchar(128) NOT NULL,
  PRIMARY KEY (`version`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_general_ci |
+-------------------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------+

[mheffner]