Fix warning in Maven build by providing version for plugins

Author: adrianshum

pom.xml

@@ -139,6 +139,7 @@
             <plugin>
                 <groupId>org.apache.maven.plugins</groupId>
                 <artifactId>maven-compiler-plugin</artifactId>
+                <version>2.3.2</version>
                 <configuration>
                     <source>1.5</source>
                     <target>1.5</target>