Consistently use Square's mockwebserver

Google mockwebserver merged into OkHttp as per:

https://code.google.com/p/mockwebserver/
It only fails here because it includes some dependencies (the `enterprise` driver) that bring BouncyCastle in. Providers that use the SSHJ driver, such as [digitalocean](https://github.com/jclouds/jclouds-labs/blob/master/digitalocean/pom.xml#L83) also have that exclusion, to avoid conflicts.

I don't know if it would be safe to move the exclusion to the `project/pom.xml`... What if no other BounCycastle dependency is present?