Simple formatter which only displays the message.

Methods
Public Instance methods
call(severity, timestamp, progname, msg)

This method is invoked when a log event occurs

     # File activesupport/lib/active_support/core_ext/logger.rb, line 116
116:     def call(severity, timestamp, progname, msg)
117:       "#{String === msg ? msg : msg.inspect}\n"
118:     end