Fix Rails4 deprecation warning: Relation#all is deprecated

DEPRECATION WARNING: Relation#all is deprecated. If you want to eager-load a relation, you can call #load (e.g. `Post.where(published: true).load`).
 If you want to get an array of records from a relation, you can call #to_a (e.g. `Post.where(published: true).to_a`).
(called from all_settings at lib/setler/settings.rb:64)
  FeatureFlag Load (3.8ms)  SELECT "feature_flags".* FROM "feature_flags" WHERE "feature_flags"."thing_type" IS NULL AND "feature_flags"."thing_id" IS NULL