Move to utf8mb4_unicode_ci for default MySQL collation

[MasterOdin]

A proposal for Phinx 0.13.x release, as this is a breaking change.

Currently, the default collation for a table is utf8_general_ci (https://github.com/cakephp/phinx/blob/master/src/Phinx/Db/Adapter/MysqlAdapter.php#L243). This is a legacy collation that has a broken utf8 implementation that should be avoided. While utf8mb4_general_ci does exist and is "faster" as it takes various shortcuts on sorting (which can resort in wrong sorts in a number of cases), this speed-up is almost negligible on any modern machine that there's no reason to suggest it. As such, it should be that utf8mb4_unicode_ci is used as the default and then for anyone who wants something different, an override option is provided.

[dereuromark]

I prepared 0.next ( https://github.com/cakephp/phinx/tree/0.next ), so we can do all those things in there now as prep for a future 0.13.