Minor refactor of ActiveRecord::SchemaMigration

This is a very minor refactor of the ActiveRecord::SchemaMigration class.

* Removed reference to ActiveRecord::Base when calling table_name_prefix and table_name_suffix.
* Override table_exists? class method to bypass schema_cache used on the ActiveRecord::Base version.
* Switched class method definitions to preferred (class << self) style.

No tests added and no documentation modified since this is a small refactor.