gemspec: Rubinius support gems

  - see https://github.com/teamcapybara/capybara/pull/1194/files#r7912212 for details

Author: olleolleolle

jbuilder.gemspec

@@ -11,6 +11,12 @@ Gem::Specification.new do |s|
 
   s.add_dependency 'activesupport', '>= 5.0.0'
 
+  if RUBY_ENGINE == 'rbx'
+    s.add_development_dependency('racc')
+    s.add_development_dependency('json')
+    s.add_development_dependency('rubysl')
+  end
+
   s.files         = `git ls-files`.split("\n")
   s.test_files    = `git ls-files -- test/*`.split("\n")
 end