Fix CLI specs

[jamesarosen]

The CLI specs are failing right now because they rely on stubbing the HTTP responses, but the stubbing happens in-process, while the calls happens within a subprocess via

`ahab fetch`

1. add rack-stubs to the test dependencies
2. spin up a Rack application in a child process that uses RackStubs::Middleware
3. change the HTTP-related steps to stub requests via POSTs to that server
4. change the CLI-related steps to refer to that server
5. ensure the subprocess is torn down before the test process exits

Some things that might be helpful:

• pid = fork do ... end
• Process.wait(pid)
• at_exit