Using mojo extra-enforcer-rules enforceBytecodeVersion.

This rule enforces a bytecode version in dependencies.
This would prevent cases like JENKINS-8914 where someone had
introduced a dependency compiled with JDK1.6 at a time where
Jenkins was still on 1.5.

With 1.7 already here and 1.8 coming, this should help no to do that
again.

pom.xml

@@ -624,10 +624,20 @@ THE SOFTWARE.
                 <requireMavenVersion>
                   <version>3.0</version>
                 </requireMavenVersion>
+                <enforceBytecodeVersion>
+                  <maxJdkVersion>1.6</maxJdkVersion>
+                </enforceBytecodeVersion>
               </rules>
             </configuration>
           </execution>
         </executions>
+        <dependencies>
+          <dependency>
+            <groupId>org.codehaus.mojo</groupId>
+            <artifactId>extra-enforcer-rules</artifactId>
+            <version>1.0-alpha-4</version>
+          </dependency>
+        </dependencies>
       </plugin>
 
       <plugin>