Gemspec: Remove duplicate dependencies

Turns out some dependencies were declared more than once, not sure why. Seems that recent Bundler versions are raising a warning about it, whereas previously it wasn't noted.

The warning this fixes is:

rack-superfeedr at /the/path 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 rack (>= 0), (>= 0) use:
    add_runtime_dependency 'rack', '>= 0', '>= 0'