Can this run on a later version of ruby?

[0xdevalias]

Currently the Gemfile pins the application to 2.1.5. We can hack around that with sed, but running on 2.4.1 ended up with the following errors:

⇒  docker run --rm -it devalias/oxml_xxe
/usr/local/bundle/gems/sinatra-1.4.7/lib/sinatra/base.rb:1226: warning: constant ::Fixnum is deprecated
/usr/local/bundle/gems/sinatra-1.4.7/lib/sinatra/base.rb:1226: warning: constant ::Fixnum is deprecated
/usr/local/bundle/gems/sinatra-1.4.7/lib/sinatra/base.rb:1226: warning: constant ::Fixnum is deprecated
/usr/local/bundle/gems/sinatra-1.4.7/lib/sinatra/base.rb:1226: warning: constant ::Fixnum is deprecated
/usr/local/lib/ruby/site_ruby/2.4.0/rubygems/specification.rb:2290:in `raise_if_conflicts': Unable to activate dm-serializer-1.2.2, because json-2.0.2 conflicts with json (~> 1.6) (Gem::ConflictError)
	from /usr/local/lib/ruby/site_ruby/2.4.0/rubygems/specification.rb:1410:in `activate'
	from /usr/local/lib/ruby/site_ruby/2.4.0/rubygems/specification.rb:1444:in `block in activate_dependencies'
	from /usr/local/lib/ruby/site_ruby/2.4.0/rubygems/specification.rb:1430:in `each'
	from /usr/local/lib/ruby/site_ruby/2.4.0/rubygems/specification.rb:1430:in `activate_dependencies'
	from /usr/local/lib/ruby/site_ruby/2.4.0/rubygems/specification.rb:1412:in `activate'
	from /usr/local/lib/ruby/site_ruby/2.4.0/rubygems.rb:220:in `rescue in try_activate'
	from /usr/local/lib/ruby/site_ruby/2.4.0/rubygems.rb:213:in `try_activate'
	from /usr/local/lib/ruby/site_ruby/2.4.0/rubygems/core_ext/kernel_require.rb:126:in `rescue in require'
	from /usr/local/lib/ruby/site_ruby/2.4.0/rubygems/core_ext/kernel_require.rb:40:in `require'
	from /oxml_xxe/model/master.rb:2:in `<top (required)>'
	from /usr/local/lib/ruby/site_ruby/2.4.0/rubygems/core_ext/kernel_require.rb:55:in `require'
	from /usr/local/lib/ruby/site_ruby/2.4.0/rubygems/core_ext/kernel_require.rb:55:in `require'
	from server.rb:11:in `<main>'

Raised https://github.com/BuffaloWill/oxml_xxe/issues/24