Add logger as a dependency

[bastelfreak]

This gem was vendored in Ruby until version 4.

$ git grep require.*logger
lib/proxy/log.rb:require 'logger'
lib/proxy/log.rb:      require 'syslog/logger'
test/log_buffer/decorator_test.rb:    require 'syslog/logger'

[ekohl]

I'd prefer to include it in #928 if this is needed (and I think you're right it is).

[bastelfreak]

Feel free to pull it into your PR :)

[ekohl]

Will do. And it's precisely the reason why testing all the dependencies is a good step :)