Ruby 2.4 Fixnum deprecation

In Ruby 2.4, Fixnum is deprecated and the following produces a warning:

$ irb
irb(main):001:0> puts RUBY_VERSION
2.4.0
=> nil
irb(main):002:0> require 'oga'
=> true
irb(main):003:0> Oga.parse_xml('<people><person>Alice</person></people>').css(':nth-child(1)')
/Users/pcheah/.rbenv/versions/2.4.0/lib/ruby/gems/2.4.0/gems/oga-2.7/lib/oga/xpath/conversion.rb:79: warning: constant ::Fixnum is deprecated
=> NodeSet(Element(name: "people" children: NodeSet(Element(name: "person" children: NodeSet(Text("Alice"))))), Element(name: "person" children: NodeSet(Text("Alice"))))
irb(main):004:0>
$

So oga/xpath/conversion.rb needs to test for Integer instead of Fixnum.

Also, it appears that json 1.8.3 no longer builds under Ruby 2.4, so the gemfile has to be upgraded to json 2.0.

lib/oga/xpath/conversion.rb

@@ -76,7 +76,7 @@ module Oga
 
         if value.is_a?(Float)
           bool = !value.nan? && !value.zero?
-        elsif value.is_a?(Fixnum)
+        elsif value.is_a?(Integer)
           bool = !value.zero?
         elsif value.respond_to?(:empty?)
           bool = !value.empty?