Make config env, root path and options read only

Not sure why these were an accessor, but they don't need to define
writers to allow app to overwrite the config after AppSignal has
started. That can cause problems and expectations that the changed
config is picked up.

[skip changeset] as these are private APIs.

lib/appsignal/config.rb

@@ -181,7 +181,7 @@ def self.determine_root_path
     }.freeze
 
     # @api private
-    attr_accessor :root_path, :env, :config_hash
+    attr_reader :root_path, :env, :config_hash
 
     # List of config option sources. If a config option was set by a source,
     # it's listed in that source's config options hash.