Remove duplicate dependency

Last bundle with last ruby complains about the duplicate dependency on rails :

````
rails-footnotes at /Users/adrien/.gem/ruby/2.1.0/bundler/gems/rails-footnotes-55871d47fd7e 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 rails (>= 3.0.0, development), (>= 3.0.0) use:
    add_runtime_dependency 'rails', '>= 3.0.0', '>= 3.0.0'
````