sdk update center: use 3.1 specific path for contributed plugins

sdk/jme3-core-updatecenters/src/com/jme3/gde/core/updatecenters/Bundle.properties

@@ -1,7 +1,7 @@
 #jMP update centers
 com_jme3_gde_core_update_center_nightly=http://updates.jmonkeyengine.org/nightly/3.1/plugins/updates.xml
 com_jme3_gde_core_update_center_stable=http://updates.jmonkeyengine.org/stable/3.1/plugins/updates.xml
-com_jme3_jmp_contributions_update_center=http://updates.jmonkeyengine.org/contributions/updates.xml
+com_jme3_jmp_contributions_update_center=http://updates.jmonkeyengine.org/contrib/3.1/updates.xml
 
 #jMP update centers
 Services/AutoupdateType/com_jme3_gde_core_update_center_nightly.instance=jMonkeyEngine SDK Nightly (Breaks!)