Upgrade to mybatis-parent 31 GA

Fixes gh-2

Author: kazuki43zoo

pom.xml

@@ -1,7 +1,7 @@
 <?xml version="1.0" encoding="UTF-8"?>
 <!--
 
-       Copyright 2018 the original author or authors.
+       Copyright 2018-2019 the original author or authors.
 
        Licensed under the Apache License, Version 2.0 (the "License");
        you may not use this file except in compliance with the License.
@@ -24,7 +24,7 @@
   <parent>
     <groupId>org.mybatis</groupId>
     <artifactId>mybatis-parent</artifactId>
-    <version>31-SNAPSHOT</version>
+    <version>31</version>
     <relativePath />
   </parent>