Add Client middleware on server configuration

From https://github.com/mperham/sidekiq/wiki/Middleware:
> Remember that the jobs running in the Sidekiq server can themselves
push new jobs to Sidekiq, thus acting as clients.
Only add/remove to chain when options change

There's no reason to add/remove the middleware from the middleware
chain everytime a sidekiq job runs. Only do it when the options change.

Fixes #7

Conflicts:
	lib/librato-sidekiq/middleware.rb
Release as thibaudgg-librato-sidekiq gem
Add bundler/gem_tasks