Add RubyRequirement.

Library/Homebrew/dependency_collector.rb

@@ -116,6 +116,7 @@ class DependencyCollector
     when :python     then PythonDependency.new(tags)
     when :python3    then Python3Dependency.new(tags)
     when :java       then JavaDependency.new(tags)
+    when :ruby       then RubyRequirement.new(tags)
     when :osxfuse    then OsxfuseDependency.new(tags)
     when :tuntap     then TuntapDependency.new(tags)
     when :ant        then ant_dep(spec, tags)

Library/Homebrew/requirements.rb

@@ -8,6 +8,7 @@ require 'requirements/mpi_dependency'
 require 'requirements/osxfuse_dependency'
 require 'requirements/python_dependency'
 require 'requirements/java_dependency'
+require 'requirements/ruby_requirement'
 require 'requirements/tuntap_dependency'
 require 'requirements/unsigned_kext_requirement'
 require 'requirements/x11_dependency'

Library/Homebrew/requirements/ruby_requirement.rb

@@ -0,0 +1,33 @@
+class RubyRequirement < Requirement
+  fatal true
+  #default_formula "ruby"
+
+  def initialize(tags)
+    @version = tags.shift if /(\d\.)+\d/ === tags.first
+    raise "RubyRequirement requires a version!" unless @version
+    super
+  end
+
+  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(@version)
+  end
+
+  env do
+    ENV.prepend_path "PATH", which("ruby").dirname
+  end
+
+  def message
+    version_string = " #{@version}" if @version
+
+    s = "Ruby#{version_string} is required to install this formula."
+    s += super
+    s
+  end
+
+  def inspect
+    "#<#{self.class.name}: #{name.inspect} #{tags.inspect} version=#{@version.inspect}>"
+  end
+end