Fix bug in find_edge_range()

This fixes issue #281.

src/graph_tool/util/__init__.py

@@ -88,7 +88,7 @@ def find_edge(g, prop, match):
 def find_edge_range(g, prop, range):
     """Find all edges `e` for which `range[0] <= prop[e] <= range[1]`. The
     parameter prop can be either a :class:`~graph_tool.PropertyMap`."""
-    convert = converter(prop.value_type())
+    convert = _converter(prop.value_type())
     ret = libgraph_tool_util.\
           find_edge_range(g._Graph__graph, _prop("e", g, prop),
                           (convert(range[0]), convert(range[1])))