feat: only allow for insights to be enabled when public (#642)

This means that the gem must be able to report data as well requiring
the environment to not be a `development_environment`. This should
prevent any unexpected Insights related code to run in dev or test
environments.

lib/honeybadger/config.rb

@@ -289,7 +289,7 @@ def load_plugin?(name)
     end
 
     def insights_enabled?
-      !!self[:'insights.enabled']
+      public? && !!self[:'insights.enabled']
     end
 
     def cluster_collection?(name)