Use Map for updating environment variables.

This ensures that the newly inserted variables overwrite any existing
variables. For more information, see:

https://groups.google.com/d/msg/yesodweb/0AX2GbKqmIk/nKYTCVoCm4QJ

yesod-bin/Devel.hs

@@ -276,7 +276,10 @@ devel opts passThroughArgs = withSocketsDo $ withManager $ \manager -> do
                    liftIO $ I.writeIORef iappPort appPort
 
                    (_,_,_,ph) <- liftIO $ createProcess (proc "runghc" devArgs)
-                        { env = Just $ ("PORT", show appPort) : ("DISPLAY_PORT", show $ develPort opts) : env0
+                        { env = Just $ Map.toList
+                                     $ Map.insert "PORT" (show appPort)
+                                     $ Map.insert "DISPLAY_PORT" (show $ develPort opts)
+                                     $ Map.fromList env0
                         }
                    derefMap <- get
                    watchTid <- liftIO . forkIO . try_ $ flip evalStateT derefMap $ do

yesod-bin/yesod-bin.cabal

@@ -1,5 +1,5 @@
 name:            yesod-bin
-version:         1.2.9.1
+version:         1.2.9.2
 license:         MIT
 license-file:    LICENSE
 author:          Michael Snoyman <michael@snoyman.com>