Backport to JDK 6

Replaced all uses of ju.Objects.hashCode and ju.Objects.equals with a
copy of their implementations inlined into the code.

Also changed source/target for compiler plugin.

To test, I compiled my own avaje launchagent against 6, and changed to
use that in the pom, compiled/tested the whole project using JDK 7,
then ran mvn surefire:test using JDK 6 - running surefire:test ensures
that mvn doesn't try to recompile everything against 6, since that's not
possible because of some of the delegate classes having delegate methods
to JDK 7 jdbc classes.

src/main/java/com/avaje/ebeaninternal/api/HashQueryPlanBuilder.java

@@ -1,7 +1,5 @@
 package com.avaje.ebeaninternal.api;
 
-import java.util.Objects;
-
 /**
  * Used to build HashQueryPlan instances.
  */
@@ -33,7 +31,7 @@ public class HashQueryPlanBuilder {
    * Add an object to the hash calculation.
    */
   public HashQueryPlanBuilder add(Object object) {
-    planHash = planHash * 31 + Objects.hashCode(object);
+    planHash = planHash * 31 + (object == null ? 0 : object.hashCode());
     return this;
   }