Allow transformers-compat 0.7 (#1436)

Builds fine and all tests pass.

Author: felixonmars

servant-client/servant-client.cabal

@@ -72,7 +72,7 @@ library
     , monad-control         >= 1.0.2.3  && < 1.1
     , semigroupoids         >= 5.3.1    && < 5.4
     , transformers-base     >= 0.4.5.2  && < 0.5
-    , transformers-compat   >= 0.6.2    && < 0.7
+    , transformers-compat   >= 0.6.2    && < 0.8
 
   hs-source-dirs: src
   default-language: Haskell2010