Simplify sponsor_levels response

This PR modifies the GET /api/events/latest/sponsors API call to:

remove the unused timestamps
return an array of sponsor_level.names, which are the only parts of that object used in the Android app

I created two sponsors and associated them with sponsor levels. Here is the output:

[
    {
        "id": 1,
        "link": "http://example.com/acme",
        "name": "Acme",
        "photo": "http://example.com/acme/photo.jpg",
        "sponsor_levels": [
            "Bronze"
        ]
    },
    {
        "id": 2,
        "link": "http://example.com/sweet",
        "name": "Sweet Company",
        "photo": "http://example.com/sweet/photo.jpg",
        "sponsor_levels": [
            "Gold"
        ]
    }
]

This is a breaking change to the mobile apps, but it simplifies things (at least for Android). Thoughts on this?

@crebma would you be interested in getting RSpec integrated into the codebase? I wanted to add a test case with this JSON schema matcher gem, but I think it assumes RSpec is the test framework.

selfconference / selfconf

Simplify sponsor_levels response #8