build: use java6+

pom.xml

@@ -47,8 +47,8 @@
                 <groupId>org.apache.maven.plugins</groupId>
                 <artifactId>maven-compiler-plugin</artifactId>
                 <configuration>
-                    <source>1.5</source>
-                    <target>1.5</target>
+                    <source>1.6</source>
+                    <target>1.6</target>
                 </configuration>
             </plugin>
             <plugin>