Require ActiveSupport::Deprecation explicitly

This commit fixes test failures with Rails 4.1.X

    Failure/Error: before { ActiveSupport::Deprecation.silence { Lograge.setup(app_config) } }
    NameError:
      uninitialized constant ActiveSupport::Deprecation
    # ./spec/lograge_spec.rb:51:in `block (3 levels) in <top
    # (required)>'

See https://travis-ci.org/roidrage/lograge/jobs/25210263#L86