Use 'dyn' on trait objects.

This fixes warnings when building with the currently supported
compilers.
diff --git a/src/dot.rs b/src/dot.rs
index 9d42b62..671338b 100644
--- a/src/dot.rs
+++ b/src/dot.rs
@@ -96,8 +96,8 @@
         where G: NodeIndexable + IntoNodeReferences + IntoEdgeReferences,
               G: GraphProp,
               G: Data<NodeWeight=NW, EdgeWeight=EW>,
-              NF: FnMut(&NW, &mut FnMut(&Display) -> fmt::Result) -> fmt::Result,
-              EF: FnMut(&EW, &mut FnMut(&Display) -> fmt::Result) -> fmt::Result,
+              NF: FnMut(&NW, &mut dyn FnMut(&dyn Display) -> fmt::Result) -> fmt::Result,
+              EF: FnMut(&EW, &mut dyn FnMut(&dyn Display) -> fmt::Result) -> fmt::Result,
     {
         if !self.config.contains(&Config::GraphContentOnly) {
             try!(writeln!(f, "{} {{", TYPE[g.is_directed() as usize]));