Fix Uninitialized Constant Error with Sinatra

The latest ActiveRecord extensions cause the gem to throw a uninitialized constant error when you use a framework other than Rails, in my case I was using Sinatra.

  /rollbar-0.10.8/lib/rollbar/active_record_extension.rb:3:in `<module:ActiveRecordExtension>': uninitialized constant ActiveSupport::Concern (NameError)

To fix the I made the active_record_extension require dependent on the ActiveRecord module being present.