Fix Ruby 2.4 deprecation warning

    warning: constant ::Fixnum is deprecated

Historically `Fixnum` and `Bignum` are specific implementations of
`Integer`. Historically, this implementation detail has been leaked
which can cause some unexpected surprises. Most of the time this won't
cause any issue. However, if the number happens to exceed the internal
`Fixnum` limitation Ruby will provide you a `Bignum`. In these case,
when neither `Bignum` nor `Integer` is referenced the code will likely
fail or cause strange behavior as the expected path is not taken.

To help people avoid this Ruby 2.4 hides these classes, as they are
just implementation details, and lets people know they should be using
`Integer`. While both the `Fixnum` and `Bignum` constants still exist
they are simply aliases of `Integer`.

Author: cupakromer

lib/kracken/json_api/public_exceptions.rb

@@ -109,7 +109,7 @@ def numeric_code(status)
         case status
         when Symbol
           code = Rack::Utils::SYMBOL_TO_STATUS_CODE[status]
-        when Fixnum
+        when Integer
           code = status
         when String
           code = status.to_i