Fix Fixnum Bignum deprecation warning

[sfcgeorge]

Ruby 2.4 unifies Fixnum and Bignum into Integer and prints a warning if the old constants are accessed. hyper-react causes this warning to be printed on boot:

lib/reactive-ruby/serializers.rb:1: warning: constant ::Bignum is deprecated
lib/reactive-ruby/serializers.rb:1: warning: constant ::Fixnum is deprecated

I've fixed this in the same way as Rails to support both older and newer versions of Ruby.