global: raise unless Ruby 2.

Library/Homebrew/global.rb

@@ -31,6 +31,7 @@ else
 end
 RUBY_BIN = RUBY_PATH.dirname
 RUBY_TWO = RUBY_VERSION.split(".").first.to_i >= 2
+raise "Homebrew must be run under Ruby 2!" unless RUBY_TWO
 
 HOMEBREW_USER_AGENT_CURL = ENV["HOMEBREW_USER_AGENT_CURL"]
 HOMEBREW_USER_AGENT_RUBY = "#{ENV["HOMEBREW_USER_AGENT"]} ruby/#{RUBY_VERSION}-p#{RUBY_PATCHLEVEL}"