Relax bound on transformers for GHC 8

yesod-auth-oauth2.cabal

@@ -38,7 +38,7 @@ library
                    , yesod-auth              >= 1.3       && < 1.5
                    , text                    >= 0.7       && < 2.0
                    , yesod-form              >= 1.3       && < 1.5
-                   , transformers            >= 0.2.2     && < 0.5
+                   , transformers            >= 0.2.2     && < 0.6
                    , hoauth2                 >= 0.4.7     && < 0.6
                    , lifted-base             >= 0.2       && < 0.4
                    , vector                  >= 0.10      && < 0.12