Let the Rails logger print backtraces

[k0kubun]

This PR changes Rails app benchmarks to avoid suppressing error backtraces by using the default STDOUT logger with log_level = :error instead of a nil logger.

It still prints nothing when you run a working Ruby implementation, but prints error backtraces when it has a bug that leads to exceptions. Since yjit-bench is a place where we often encounter new bugs in JITs, we should be able to see backtraces without modifying the source code.