Allow lens 4.19

https://github.com/ekmett/lens/blob/v4.19.1/CHANGELOG.markdown

Author: tfausak

servant-foreign/servant-foreign.cabal

@@ -57,7 +57,7 @@ library
   -- Here can be exceptions if we really need features from the newer versions.
   build-depends:
       base-compat >= 0.10.5  && < 0.12
-    , lens        >= 4.17    && < 4.19
+    , lens        >= 4.17    && < 4.20
     , http-types  >= 0.12.2  && < 0.13
 
   hs-source-dirs:      src