pom.xml: specify minimum Maven version in prerequisites.

Suggestion from `mvn versions:display-plugin-updates`

No functional changes.

Author: php-coder

pom.xml

@@ -606,6 +606,10 @@
 		</license>
 	</licenses>
 
+	<prerequisites>
+		<maven>2.2.1</maven>
+	</prerequisites>
+	
 	<scm>
 		<connection>scm:git:git://github.com/php-coder/mystamps.git</connection>
 		<url>https://github.com/php-coder/mystamps</url>