conflict with rspec and rspec-spies

The development dependency in jenkins-plugin-runtime.gemspec has rspec not locked down so a fresh bundle might give you rspec 2.14 which has rspec-spies included. This causes specs to fail because mocks are called twice.

```
Failures:

  1) Jenkins::Plugin registering an extension. when no order is defined uses 0
     Failure/Error: @peer.should have_received(:addExtension).with(@ext, 0)
       (Mock).addExtension(#<Object:0x21f9277b>, 0)
           expected: 1 time with arguments: (#<Object:0x21f9277b>, 0)
           received: 2 times with arguments: (#<Object:0x21f9277b>, 0)
     # ./spec/jenkins/plugin_spec.rb:45:in `(root)'
```

We can either lock the rspec version or update the gemspec and remove rspec-spies. I'd be happy to submit a pull request for either.