mysql-native v2.2.2: Minor updates

Fri May 4 02:59:10 UTC 2018

An all-D MySQL/MariaDB client library:
https://github.com/mysql-d/mysql-native
==========================================

Tagged release, 'v2.2.2'.

Sorry, no "zero date" fix just yet, but I think we're getting close (#65).

In this version:
- Fixed #177: Update unit-threaded, and don't lock mysql-native to a 
specific version of unit-threaded. (@ghost91)
- Fixed #178, #179: Fix deprecation message in DMD v2.080.0: Replace 
enforceEx by enforce (@ghost91)

Full changelog:
https://github.com/mysql-d/mysql-native/blob/master/CHANGELOG.md