Fix Delayed::Job <-> Rails 4.1 dependency conflict

Fixes:

    Bundler could not find compatible versions for gem "activerecord":
      In Gemfile:
        delayed_job_active_record (>= 4.0.0) ruby depends on
          activerecord (< 4.1, >= 3.0) ruby

        rails (>= 4.0.3) ruby depends on
          activerecord (4.1.0)

https://github.com/collectiveidea/delayed_job_active_record/pull/86