Fix Problem with mkdir

After upgrading my ruby & rake, the app template stopped working on a build, failing at making the deploy directory.  This change adds Dir before mkdir and now I don't get this error any more:

jmartin-mbp:AppTemplate jmartin$ rake build
Building App...
rake aborted!
undefined method `mkdir' for #<Rally::AppSdk::AppTemplateBuilder:0x007fb43c10c668>
/Users/jmartin/views/rally/git/sdk2/github/rally/AppTemplate/Rakefile:568:in `assure_deploy_directory_exists'
/Users/jmartin/views/rally/git/sdk2/github/rally/AppTemplate/Rakefile:537:in `build_app_html'
/Users/jmartin/views/rally/git/sdk2/github/rally/AppTemplate/Rakefile:31:in `block in <top (required)>'
/Users/jmartin/.rvm/gems/ruby-1.9.2-p320/bin/ruby_noexec_wrapper:14:in `eval'
/Users/jmartin/.rvm/gems/ruby-1.9.2-p320/bin/ruby_noexec_wrapper:14:in `<main>'
Tasks: TOP => build
(See full trace by running task with --trace)