Allow attoparsec 0.14 (#1408)

Builds fine and all tests pass.

Author: felixonmars

servant/servant.cabal

@@ -97,7 +97,7 @@ library
   build-depends:
       base-compat            >= 0.10.5   && < 0.12
     , aeson                  >= 1.4.1.0  && < 1.6
-    , attoparsec             >= 0.13.2.2 && < 0.14
+    , attoparsec             >= 0.13.2.2 && < 0.15
     , bifunctors             >= 5.5.3    && < 5.6
     , case-insensitive       >= 1.2.0.11 && < 1.3
     , deepseq                >= 1.4.2.0  && < 1.5