SDK Update Center URL: change 3.0 -> 3.1

Doesn't work yet, because the 3.1 folder is missing from the updates server.

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

@@ -1,6 +1,6 @@
 #jMP update centers
-com_jme3_gde_core_update_center_nightly=http://updates.jmonkeyengine.org/nightly/3.0/plugins/updates.xml
-com_jme3_gde_core_update_center_stable=http://updates.jmonkeyengine.org/stable/3.0/plugins/updates.xml
+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
 
 #jMP update centers