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/server/expression/AllEqualsExpression.java

@@ -3,7 +3,6 @@ package com.avaje.ebeaninternal.server.expression;
 import java.util.Iterator;
 import java.util.Map;
 import java.util.Map.Entry;
-import java.util.Objects;
 import java.util.Set;
 
 import com.avaje.ebean.event.BeanQueryRequest;
@@ -115,7 +114,7 @@ class AllEqualsExpression implements SpiExpression {
     
     int hc = 31;
     for (Object value : propMap.values()) {
-      hc = hc * 31 + Objects.hashCode(value);      
+      hc = hc * 31 + (value == null ? 0 : value.hashCode());
     }
 
     return hc;