Loosen the rdoc gem dependency to be compatible with Rails

Rails uses rdoc 3.4

The fastly-ruby gem uses rdoc >= 3.11

Which means can't update to Rails 3.2.15 because of the gem dependency issue

Bundler could not find compatible versions for gem "rdoc":
  In Gemfile:
    rails (= 3.2.15) ruby depends on
      rdoc (~> 3.4) ruby

    fastly (~> 1.01) ruby depends on
      rdoc (4.0.1)