How would I write this differently to make it compatible with MYSQL 5.7
Turns out this is a bug as of Craft 2.6.2931 with the ColumnType::PK enum 
in a record's attribute type definition.

Pretty sure it'd be a bug in &lt; MySQL 5.7 because it's invalid SQL syntax as well unless they're just more forgiving of it there.

This is a non-issue in the upcoming Craft 3 release (the concept of attributes no longer exists and plugins explicitly define their table schema create statements), but the workaround for Craft 2 until the bug is fixed is to not use a record here and manually create the table schema that you need by adding an onBeforeInstall method to your plugin.