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/HashQueryPlan.java

@@ -1,7 +1,5 @@
 package com.avaje.ebeaninternal.api;
 
-import java.util.Objects;
-
 /**
  * A hash for a query plan.
  */
@@ -26,7 +24,7 @@ public class HashQueryPlan {
   public int hashCode() {
     int hc = planHash;
     hc = hc * 31 + bindCount;
-    hc = hc * 31 + Objects.hashCode(rawSql);
+    hc = hc * 31 + (rawSql == null ? 0 : rawSql.hashCode());
     return hc;
   }
   
@@ -41,6 +39,6 @@ public class HashQueryPlan {
     HashQueryPlan e = (HashQueryPlan) obj;
     return e.planHash == planHash 
         && e.bindCount == bindCount
-        && Objects.equals(e.rawSql, rawSql);
+        &&  ((e.rawSql == rawSql) || (e.rawSql != null && e.rawSql.equals(rawSql)));
   }
 }