[ruby/fileutils] Fix test failure under ruby 2.4
Exception#full_message is only defined on ruby 2.5.0 and above.
Exception#full_message
https://github.com/ruby/fileutils/commit/a8968f41ed
[ruby/fileutils] Fix test failure under ruby 2.4
Exception#full_message
is only defined on ruby 2.5.0 and above.https://github.com/ruby/fileutils/commit/a8968f41ed