Fix rake dependency in gemspec

Error was (in ruby 2.1.0):
    quarto at /Users/brad/dev/avdi/quarto did not have a valid gemspec.
    This prevents bundler from installing bins or native extensions, but
    that may not affect its functionality.
    The validation message from Rubygems was:
      duplicate dependency on rake (~> 10.0, development), (~> 10.0) use:
          add_runtime_dependency 'rake', '~> 10.0', '~> 10.0'