Use the correct unified backend name

FilipeMaia · FilipeMaia · commit 30a3d05a3467 · 2016-03-30T17:36:28.000+02:00
diff --git a/arrayfire/library.py b/arrayfire/library.py
@@ -507,7 +507,7 @@ def name(self):
         return self.__name
 
     def is_unified(self):
-        return self.__name == ''
+        return self.__name == 'unified'
 
     def parse(self, res):
         lst = []
