diff --git a/.travis.yml b/.travis.yml
index 67405dc..ea016eb 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -2,12 +2,15 @@ env: CI=1
 
 rvm:
   - 1.8.7
-  - 1.9.1
   - 1.9.2
-  - jruby
-  - rbx
+  - 1.9.3
+  - jruby-18mode
+  - jruby-19mode
+  - rbx-18mode
+  - rbx-19mode
   - ree
   - ruby-head
+  - jruby-head
 
 notifications:
   disabled: true
diff --git a/Gemfile b/Gemfile
index 1a879dc..fb1af24 100644
--- a/Gemfile
+++ b/Gemfile
@@ -5,7 +5,9 @@ gemspec
 group :test do
   gem 'johnson',      :platform => :mri_18
   gem 'json'
-  gem 'mustang',      :platform => :ruby
+  # see https://github.com/nu7hatch/mustang/issues/18
+  gem 'mustang',      :platform => :ruby,
+    :git => "https://github.com/nu7hatch/mustang.git", :ref => "2a3bcfbd9fd0f34e9b004fcd92188f326b40ec2a"
   gem 'therubyracer', :platform => :ruby
   gem 'therubyrhino', :platform => :jruby
 end
diff --git a/Rakefile b/Rakefile
index cf8fe47..c29f78d 100644
--- a/Rakefile
+++ b/Rakefile
@@ -7,7 +7,7 @@ require "execjs/runtimes"
 
 tests = namespace :test do |tests|
   ExecJS::Runtimes.names.each do |name|
-    next if name == :Disabled
+    next if (name == :Disabled || name == "Disabled")
 
     task(name.downcase) do
       ENV["EXECJS_RUNTIME"] = name.to_s
diff --git a/execjs.gemspec b/execjs.gemspec
index 30242ce..29986fb 100644
--- a/execjs.gemspec
+++ b/execjs.gemspec
@@ -7,18 +7,24 @@ Gem::Specification.new do |s|
 
   s.homepage    = "https://github.com/sstephenson/execjs"
   s.summary     = "Run JavaScript code from Ruby"
-  s.description = <<-EOS
-    ExecJS lets you run JavaScript code from Ruby.
-  EOS
+  s.description = "ExecJS lets you run JavaScript code from Ruby."
 
   s.files = Dir["README.md", "LICENSE", "lib/**/*"]
 
   s.add_dependency "multi_json", "~>1.0"
-  s.add_development_dependency "johnson"
-  s.add_development_dependency "mustang"
   s.add_development_dependency "rake"
-  s.add_development_dependency "therubyracer"
-  s.add_development_dependency "therubyrhino"
+
+  if RUBY_VERSION < "1.9" && (!defined?(RUBY_ENGINE) || RUBY_ENGINE == "ruby")
+    # see https://github.com/jbarnette/johnson/issues/21
+    s.add_development_dependency "johnson"
+  end
+
+  if (!defined?(RUBY_ENGINE) || RUBY_ENGINE != "jruby")
+    s.add_development_dependency "mustang"
+    s.add_development_dependency "therubyracer"
+  else
+    s.add_development_dependency "therubyrhino"
+  end
 
   s.authors = ["Sam Stephenson", "Josh Peek"]
   s.email   = ["sstephenson@gmail.com", "josh@joshpeek.com"]