Make the MySQL checkit() function deterministic.

Because it is, and @doy tells me that this gets it to install properly on
MariaDB. Ref Issue #158.

Changes

@@ -19,6 +19,9 @@ Revision history for Perl extension App::Sqitch
      - Fix issue where, on first deploy, the MySQL engine would fail to notice
        that the server was not the right version of MySQL. Thanks to Luke
        Young for the report (Issue #158).
+     - Made the `checkit()` MySQL function DETERMINISTIC, to improve
+       compatability with MariaDB. Thanks to Jesse Luehrs for the report
+       (Issue #158).
 
 0.992  2014-03-05T00:34:49Z
      - Fixed target test failures on Windows.

lib/App/Sqitch/Engine/mysql.sql

@@ -137,7 +137,7 @@ CREATE TABLE events (
 
 DELIMITER |
 
-CREATE FUNCTION checkit(doit INTEGER, message VARCHAR(256)) RETURNS INTEGER
+CREATE FUNCTION checkit(doit INTEGER, message VARCHAR(256)) RETURNS INTEGER DETERMINISTIC
 BEGIN
     IF doit IS NULL OR doit = 0 THEN
         SIGNAL SQLSTATE 'ERR0R' SET MESSAGE_TEXT = message;