Fix Fixnum Bignum deprecation warning

lib/reactive-ruby/serializers.rb

@@ -1,7 +1,13 @@
-[Bignum, FalseClass, Fixnum, Float, Integer, NilClass, String, Symbol, Time, TrueClass].each do |klass|
-  klass.send(:define_method, :react_serializer) do 
-    as_json
-  end
+[TrueClass, FalseClass, NilClass, Float, String, Symbol, Time].each do |klass|
+  klass.send(:define_method, :react_serializer) { as_json }
+end
+# Ruby 2.4 unifies Fixnum and Bignum into Integer
+# and prints a warning if the old constants are accessed.
+if 0.class == Integer
+  Integer.send(:define_method, :react_serializer) { as_json }
+else
+  Fixnum.send(:define_method, :react_serializer) { as_json }
+  Bignum.send(:define_method, :react_serializer) { as_json }
 end
 
 BigDecimal.send(:define_method, :react_serializer) { as_json } rescue nil