Merge pull request #10549 from mikemcquaid/brew-cask-ruby20

brew-cask: depend on Ruby 2.0.

brew-cask.rb

@@ -7,6 +7,27 @@ rescue
   HBC_VERSION = HOMEBREW_CASK_VERSION
 end
 
+class Ruby20 < Requirement
+  fatal true
+  default_formula "ruby"
+
+  satisfy :build_env => false do
+    next unless which "ruby"
+    version = /\d\.\d/.match `ruby --version 2>&1`
+    next unless version
+    Version.new(version.to_s) >= Version.new("2.0")
+  end
+
+  env do
+    ENV.prepend_path "PATH", which("ruby").dirname
+  end
+
+  def message; <<-EOS.undent
+    brew-cask needs Ruby >=2.0
+    EOS
+  end
+end
+
 class BrewCask < Formula
   homepage "https://github.com/caskroom/homebrew-cask/"
   url "https://github.com/caskroom/homebrew-cask.git", :tag => "v#{HBC_VERSION}"
@@ -14,6 +35,8 @@ class BrewCask < Formula
 
   skip_clean "bin"
 
+  depends_on Ruby20
+
   def install
     man1.install "doc/man/brew-cask.1"
     prefix.install "lib" => "rubylib"