Specify minimum maven version in pom.xml

Change-Id: Iadd13e66c901528df77a10174420a6a2c3fa2878

pom.xml

diff --git a/pom.xml b/pom.xml
index 8ee2fe5..efb64cb 100644
--- a/pom.xml
+++ b/pom.xml
@@ -26,6 +26,10 @@
   <description>Tiny ORM</description>
   <url>https://gerrit.googlesource.com/gwtorm</url>
 
+  <prerequisites>
+    <maven>3.0</maven>
+  </prerequisites>
+
   <scm>
     <url>https://gerrit.googlesource.com/gwtorm</url>
     <connection>https://gerrit.googlesource.com/gwtorm</connection>