Fixing diagnose when Rails is not present

@TheMuppet found this issue when is trying to load the appsignal, The
gem is loading rails and also verifying the application.rb path. Some
applications has that configuration already but that not means that they
are Rails already.

He proposed to fix this verifying if Rails is defined or not, with this
fix he solved the load issue.

lib/appsignal/cli/diagnose.rb

@@ -583,11 +583,7 @@ def print_empty_line
         end
 
         def rails_app?
-          require "rails"
-          require File.expand_path(File.join(Dir.pwd, "config", "application.rb"))
-          true
-        rescue LoadError
-          false
+          defined?(Rails)
         end
       end
     end

lib/appsignal/version.rb

@@ -1,5 +1,5 @@
 # frozen_string_literal: true
 
 module Appsignal
-  VERSION = "2.11.8".freeze
+  VERSION = "2.11.9".freeze
 end