Properly use Maven versioning for better support of dependent plugins

Closes #59
Bumped version to 3.1.0-SNAPSHOT for development, really should have bumped the minor version a while back.

pom.xml

@@ -6,13 +6,12 @@
   <artifactId>openinvparent</artifactId>
   <name>OpenInvParent</name>
   <url>http://dev.bukkit.org/bukkit-plugins/openinv/</url>
-  <version>1.0-SNAPSHOT</version>
+  <version>3.1.0-SNAPSHOT</version>
 
   <packaging>pom</packaging>
 
   <properties>
     <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
-    <openinv.version>3.0.7-SNAPSHOT</openinv.version>
   </properties>
 
   <modules>