Upgrade to RSpec 3.0

I'm seeing some spec failures:

    1) Docker::Container#copy when the file does not exist raises an error FIXED
       Expected pending 'Docker no longer returns a 500 when the file does not exist' to fail. No Error was raised.
       # ./spec/docker/container_spec.rb:119

    2) Docker#authenticate! with valid credentials logs in and sets the creds
       Failure/Error: expect(authentication).to be_true
         expected true to respond to `true?`
       # ./spec/docker_spec.rb:147:in `block (4 levels) in <top (required)>'

    3) Docker#validate_version when nothing is raised validate_version! should be true
       Failure/Error: its(:validate_version!) { should be_true }
         expected true to respond to `true?`
       # ./spec/docker_spec.rb:190:in `block (4 levels) in <top (required)>'

The first one if because the code after the pending mark is actually passing when RSpec 3 expects it to fail.

I'm not sure what's going on with the others.