Override getRetentionStrategy() in BeforeHourWrapsPolicy

For some reason, value maintained by hudson.model.Slave is null under
cirsumstances described in #56, which is causing return value to be
set to RetentionStrategy.Always.INSTANCE.

Closes #56

Signed-off-by: Richard Kosegi <richard.kosegi@gmail.com>

src/main/java/cloud/dnation/jenkins/plugins/hetzner/shutdown/BeforeHourWrapsPolicy.java

@@ -41,6 +41,11 @@ public BeforeHourWrapsPolicy() {
         super(STRATEGY_SINGLETON);
     }
 
+    @SuppressWarnings("rawtypes")
+    @Override
+    public RetentionStrategy<AbstractCloudComputer> getRetentionStrategy() {
+        return STRATEGY_SINGLETON;
+    }
 
     @SuppressWarnings("rawtypes")
     private static class RetentionStrategyImpl extends RetentionStrategy<AbstractCloudComputer> {