[RAILS] Fixed 03-expert.rb set tracer only in dev env (elastic#621)

The variable `tracer` is only defined in Rails development env, so we only can set them in that env.

Author: timoschilling

elasticsearch-rails/lib/rails/templates/03-expert.rb

@@ -270,9 +270,8 @@ def index
 if Rails.env.development?
   tracer = ActiveSupport::Logger.new('log/elasticsearch.log')
   tracer.level =  Logger::DEBUG
+  Elasticsearch::Model.client.transport.tracer = tracer
 end
-
-Elasticsearch::Model.client.transport.tracer = tracer
 CODE
 
 git add:    "config/initializers"