Handle printing queries with integer arguments

The GitHub v4 API uses integer argument for paging. Without this patch,
the code would attempt to run (essentially) `"last" + ": " + 10`, which
isn't valid in Python.

With this change, an argument value of `.args(last=10)` is correctly pretty-printed
as `(last: 10)`.

Author: cpennington

graphql/language/printer.py

@@ -47,7 +47,7 @@ def leave_Field(self, node, *args):
         ], ' ')
 
     def leave_Argument(self, node, *args):
-        return node.name + ': ' + node.value
+        return '{0.name}: {0.value}'.format(node)
 
     # Fragments