Respect Ruby's verbosity conventions

By Ruby convention, when `ENV['VERBOSE'] == nil`, code should be silent. The default is for `ENV['VERBOSE']` is `false`, which means that for a normal run this branch will still properly show deprecation warnings. There is a really good article on this whole [verbosity issue here](http://mislav.uniqpath.com/2011/06/ruby-verbose-mode/).

As described in #205, I'm using Webmock and am getting output during my test suite run from Excon that I can't control without extreme measures like redefining `$stderr`.

Thoughts?