Make Nokogiri version less restrictive

Fixes versioning issues:

    ideal-mollie (>= 0) ruby depends on
      nokogiri (~> 1.4.1) ruby

    capybara-webkit (>= 0) ruby depends on
      nokogiri (1.6.0)
Merge pull request #12 from tijmenb/feature/nokogiri-version

Nokogiri-versie minder pessimistisch