Added the option to flush immediately

We tried using this gem, but would not get any updates to Librato for sidekiq related metrics. We played around with it for a while, and the only thing we found that helped was manually forcing a flush at the end of the middleware after the librato calls. I see that librato has a flush interval, and I wasn't able to select a value for that that resulted in sidekiq sending the metrics to librato. I wouldn't want that number to be low anyway, since I want to use the librato-rails instrumentation throughout the app without slowing things down.

In case it helps at all, I'm using sidekiq 2.11.2, sidekiq-pro 1.0.0, ruby 1.9.3, rails 3.2.12, librato-metrics 1.1.1, and librato-rails 0.10.0.

I implemented this as an option so that it won't change the default behavior of the middleware. If you observe that this is a permanent bug caused by some newer version of something, you may want to default it to true.