Support RSpec 3

This change allows to use serverspec with RSpec 3. Of course, the support for RSpec 2 is kept.

Fortunately serverspec is not using any of [deprecated API of the custom matcher DSL in RSpec 3](https://github.com/yujinakayama/transpec/blob/v1.6.1/README.md#custom-matcher-dsl), so no change is required in serverspec.