GUACAMOLE-314: Merge extension API sanity check version number bump.

guacamole/src/main/java/org/apache/guacamole/extension/ExtensionModule.java

@@ -59,7 +59,7 @@ public class ExtensionModule extends ServletModule {
     private static final List<String> ALLOWED_GUACAMOLE_VERSIONS =
         Collections.unmodifiableList(Arrays.asList(
             "*",
-            "0.9.12-incubating"
+            "0.9.13-incubating"
         ));
 
     /**