Fix some cabal warnings

Signed-off-by: Jan Hamal Dvořák <mordae@anilinux.org>

hikaru.cabal

@@ -29,6 +29,8 @@ source-repository head
     location: https://github.com/mordae/hikaru
 
 common common
+    default-language: Haskell2010
+
     default-extensions:
         AllowAmbiguousTypes
         BlockArguments
@@ -123,30 +125,6 @@ test-suite spec
         Hikaru.Test
         Paths_hikaru
 
-    default-extensions:
-        AllowAmbiguousTypes
-        BlockArguments
-        DataKinds
-        DefaultSignatures
-        DeriveGeneric
-        EmptyDataDecls
-        FlexibleContexts
-        FlexibleInstances
-        GADTs
-        GeneralizedNewtypeDeriving
-        ImportQualifiedPost
-        KindSignatures
-        LambdaCase
-        MultiParamTypeClasses
-        NamedFieldPuns
-        NoImplicitPrelude
-        OverloadedStrings
-        RankNTypes
-        RecordWildCards
-        ScopedTypeVariables
-        StandaloneDeriving
-        UndecidableInstances
-
     ghc-options:
         -threaded
         -rtsopts