Remove deprecated Fixnum message

[dvdchr]

With Ruby 2.4.0, loading pbxplorer invokes the message:

pbxplorer.rb:7: warning: constant ::Fixnum is deprecated

I admit this small fix is a bit hacky – it checks for current RUBY_VERSION, and overrides Integer only if Ruby has version ≥2.4.0. Tests are all green.

Let me know what you think!