Error installing backlog on redmine

[root@localhostredmine]# bundle exec rakeredmine:backlogs:install

[!] There was an error parsing `Gemfile`: 
[!] There was an error parsing `Gemfile`: You cannot specify the same gem twice with different version requirements.
You specified: nokogiri(>=1.6.7.1) and nokogiri(>=0).Bundler cannot continue.

 #  from/var/www/redmine/plugins/redmine_backlogs/Gemfile: 13
 #  -------------------------------------------
 #  gem "icalendar"
 >gem "nokogiri"
 #  gem "open-uri-cached"
 #  -------------------------------------------
. Bundler cannot continue.

 #  from/var/www/redmine/Gemfile: 113
 #  -------------------------------------------
 #  Dir.globFile.expand_path("../plugins/*/{Gemfile, PluginGemfile}",__FILE__)do|file|
 >eval_gemfile file
 #  end
 #  -------------------------------------------

[policit@localhostredmine] $bundle exec rakeredmine:backlogs:install
 Could not find gem'capybara(~>1.1.0) 'in any of the gem sources
 listed in your Gemfile or available on this machine.Run bundle
 install to install missing gems.