remove ActiveSupport::Concern warnings when including MethodTracer

In Rails 3.2+, when including NewRelic::Agent::MethodTracer
and any concern based on ActiveSupport::Concern in the same class,
ActiveSupport::Concern is displaying warnings like:

    DEPRECATION WARNING: The InstanceMethods module inside ActiveSupport::Concern will be no longer included automatically. Please define instance methods directly in TheClass instead.

For `TheClass` defined as:

    class TheClass
      include SomeConcern
      include ::NewRelic::Agent::MethodTracer

      add_method_tracer :some_method
    end

given the ancestors of `TheClass`:

    [TheClass,
    NewRelic::Agent::MethodTracer::InstanceMethods,
    NewRelic::Agent::MethodTracer,
    Object, Kernel]

there’s no need for a `NewRelic::Agent::MethodTracer::InstanceMethods`
since methods on `NewRelic::Agent::MethodTracer` are already in the chain.

http://engineering.appfolio.com/2013/06/17/ruby-mixins-activesupportconcern/