method and tests to add an instance to an environment

Despite the name, this PR adds functionality to the project to add instances of *either* app or util to a given environment. (I started thinking, "I'll focus on the simplest use case - app - and go from there", but it turned out being a no-brainer for utils too).

Usage example:

```
api = EY::CloudClient.new(token: 'token')
e = (api.environments.select { |x| x.name == 'something' }).first
e.add_instance(role: 'util', name: 'elasticsearch')
e.add_instance(role 'app')
```

Returns the API response body (JSON).

Specs are included and all 77 examples pass.

Added the "pry" development dependency to make digging into things during spec execution easier.