BUG: Fixes calls to slicer.util.moduleNames()

The moduleNames() function was renamed to "loadedModuleNames"

See https://github.com/Slicer/Slicer/blob/master/Base/QTCore/qSlicerModuleFactoryManager.h#L60
    na-mic.org/Mantis/view.php?id=2591

Fixes #2591

Base/Python/slicer/util.py

@@ -240,7 +240,7 @@ def selectModule(module):
 
 def moduleNames():
   from slicer import app
-  return app.moduleManager().factoryManager().moduleNames()
+  return app.moduleManager().factoryManager().loadedModuleNames()
 
 def getModule(moduleName):
   from slicer import app