Create the GEM_HOME directory before setting it

When Bundler sets GEM_HOME before creating the path in question, JRuby's monkeypatch to Gem.set_paths ignores the directory because it doesn't exist.

There is a JRuby bug (and patch) at http://jira.codehaus.org/browse/JRUBY-5104, but it won't be included in JRuby until 1.5.4 at the earliest.

Closes #602, #734

lib/bundler.rb

@@ -253,6 +253,7 @@ def configure_gem_home_and_path
         ENV["GEM_HOME"] = bundle_path.to_s
       end
 
+      FileUtils.mkdir_p bundle_path.to_s
       Gem.clear_paths
     end