Disable full_backtrace on CI

It's too verbose and not very helpful in common situations. It's more
practical to be turn on only when it is necessary to avoid very long
backtrace output.

spec/spec_helper.rb

@@ -66,7 +66,7 @@ RSpec.configure do |config|
   config.display_try_failure_messages = true
 
   config.infer_spec_type_from_file_location!
-  config.full_backtrace = !!ENV['CI']
+  # config.full_backtrace = true
 
   unless ENV['CI']
     # Re-run failures locally with `--only-failures`