Allow Ruby 2.0.0

With Ruby 2.0.0, this gives:

    Installing qu_mailer (0.1.0) 
    Gem::InstallError: qu_mailer requires Ruby version ~> 1.9.
    An error occurred while installing qu_mailer (0.1.0), and Bundler cannot continue.
    Make sure that `gem install qu_mailer -v '0.1.0'` succeeds before bundling.

I assume the version constraint was to stop Ruby 1.8, so i've made it > 1.9.