Add Homebrew.install_gem_setup_path! function.

This uses the logic from tests.rb in man.rb too so that this can be
shared in a few places.

Library/Homebrew/cmd/man.rb

@@ -13,7 +13,7 @@ module Homebrew
       end
     end
 
-    which("ronn") || odie("You need to \"gem install ronn\" and put it in your path.")
+    Homebrew.install_gem_setup_path! "ronn"
 
     if ARGV.include?("--server") || ARGV.include?("-s")
       puts "Man page test server: http://localhost:1207/"

Library/Homebrew/cmd/tests.rb

@@ -1,12 +1,8 @@
 module Homebrew
   def tests
-    (HOMEBREW_LIBRARY/'Homebrew/test').cd do
-      ENV['TESTOPTS'] = '-v' if ARGV.verbose?
-      quiet_system("gem", "list", "--installed", "bundler") || \
-        system("gem", "install", "--no-ri", "--no-rdoc",
-               "--user-install", "bundler")
-      require 'rubygems'
-      ENV["PATH"] = "#{Gem.user_dir}/bin:#{ENV["PATH"]}"
+    (HOMEBREW_LIBRARY/"Homebrew/test").cd do
+      ENV["TESTOPTS"] = "-v" if ARGV.verbose?
+      Homebrew.install_gem_setup_path! "bundler"
       quiet_system("bundle", "check") || \
         system("bundle", "install", "--path", "vendor/bundle")
       system "bundle", "exec", "rake", "test"

Library/Homebrew/utils.rb

@@ -119,6 +119,14 @@ module Homebrew
   def self.git_last_commit
     HOMEBREW_REPOSITORY.cd { `git show -s --format="%cr" HEAD 2>/dev/null`.chuzzle }
   end
+
+  def self.install_gem_setup_path! gem
+    return if quiet_system "gem", "list", "--installed", gem
+    system "gem", "install", "--no-ri", "--no-rdoc",
+           "--user-install", gem
+    require "rubygems"
+    ENV["PATH"] = "#{Gem.user_dir}/bin:#{ENV["PATH"]}"
+  end
 end
 
 def with_system_path