Logger should not be mutated

[choubacha]

When we pass the logger to the client we end up changing the log level:

https://github.com/taskrabbit/waistband/blob/master/lib/waistband/index.rb#L279

The issue arises that this is a mutation on a logger instance that has not been defensively copied, therefore we're changing the logger for every instance.

This is likely not threadsafe.