Picasso.with() should be synchronized

I was using Picasso `2.2.0` with OkHttp `1.3.0` and everything was working fine.
I updated OkHttp to `1.5.4` and I started seeing instances of https://github.com/square/okhttp/issues/680

After researching a bit I noticed that I had two Picasso instances in my app generated by a race condition in the method that generates the Picasso singleton. I am using Picasso from the main thread but also from a background service to prefetch images.

Apparently I had this bug before but, as @swankjesse explains in https://github.com/square/okhttp/pull/650, the exception was masked by the fact that the cleanup callable was being run with a `Future`.