diff --git a/.travis.yml b/.travis.yml
index 0406e44..f66088c 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -7,10 +7,10 @@ rvm:
   - 2.7.0
   - ruby-head
   - jruby-19mode
-  - rbx-2
+  - rbx-4
 matrix:
   allow_failures:
     - rvm: ruby-head
     - rvm: jruby-19mode
-    - rvm: rbx-2
+    - rvm: rbx-4
     - rvm: 2.4
diff --git a/zhongwen_tools.gemspec b/zhongwen_tools.gemspec
index d2c3ae4..c6216e8 100644
--- a/zhongwen_tools.gemspec
+++ b/zhongwen_tools.gemspec
@@ -15,17 +15,19 @@ Gem::Specification.new do |s|
   s.test_files    = `git ls-files -- {test,spec,features}/*`.split("\n")
   s.executables   = `git ls-files -- bin/*`.split("\n").map{ |f| File.basename(f) }
   s.require_paths = ['lib']
+  s.required_ruby_version = '>= 1.9.3'
 
+  s.add_development_dependency('simplecov', '~> 0.16', '>= 0.16.0')
+  s.add_development_dependency('simplecov-gem-adapter', '~> 1.0', '>= 1.0.1')
+  s.add_development_dependency('coveralls', '~> 0.8', '>= 0.8.1')
+  s.add_development_dependency('minitest', '~> 5.5', '>= 5.5.1')
+  s.add_development_dependency('minitest-reporters', '~> 1.0', '>= 1.0.10')
   if RUBY_VERSION >= '2.1'
-    s.add_development_dependency('simplecov', '~> 0.16', '>= 0.16.0')
-    s.add_development_dependency('simplecov-gem-adapter', '~> 1.0', '>= 1.0.1')
-    s.add_development_dependency('coveralls', '~> 0.8', '>= 0.8.1')
-    s.add_development_dependency('minitest', '~> 5.5', '>= 5.5.1')
-    s.add_development_dependency('minitest-reporters', '~> 1.0', '>= 1.0.10')
     s.add_development_dependency "rake", ">= 12.3.3"
     s.add_development_dependency('pry', '~> 0.13', '>= 0.13.0')
     s.add_development_dependency('memory_profiler', '0.0.4')
   else
+    s.add_development_dependency('json', '~> 2.2.0')
     s.add_development_dependency('rake', '~> 10.1')
   end
 end