Constrain dependecy of `ipython-kernel` to actually match the version.

Previously, `cabal install ihaskell` would happily use ipython-kernel-0.8.3, resulting in error

    ‘protocolVersion’ is not a (visible) field of constructor ‘KernelInfoReply’

ihaskell.cabal

@@ -87,7 +87,7 @@ library
                        utf8-string          -any,
                        uuid                 >=1.3,
                        vector               -any,
-                       ipython-kernel       >=0.7
+                       ipython-kernel       >=0.8.4
   if flag(binPkgDb)
     build-depends:       bin-package-db