Usage of `ChronicDuration` is not thread-safe

Time conversion values are stored as class variables and we're switching them in https://gitlab.com/gitlab-org/gitlab-ce/blob/master/lib/gitlab/time_tracking_formatter.rb#L27.

This could lead to problems when multiple threads are changing the same class variable.

Since it looks like the gem is unmaintained and it's only a single class, it may be good to just copy this class over in lib/ and change it. I think we can change this implementation to use an instance where we pass the config values in the constructor.