Initialize NewRelic before Rails' config-initializers

Before, the initializer for NewRelic did not specify any ordering. In
some situations, Rails will order its load_config_initializers before
NewRelic's newrelic_rpm.start_plugin. This is a problem if NewRelic is
called in a config-initializer (e.g. when defining method tracers) and
will output the following warning:

  Agent unavailable as it hasn't been started.

This change ensures that newrelic_rpm.start_plugin always gets loaded
before config-intiializers are.

lib/newrelic_rpm.rb

@@ -37,7 +37,7 @@ def self.run
     module NewRelic
       class Railtie < Rails::Railtie
 
-        initializer "newrelic_rpm.start_plugin" do |app|
+        initializer "newrelic_rpm.start_plugin", before: :load_config_initializers do |app|
           NewRelic::Control.instance.init_plugin(:config => app.config)
         end
       end