Use ssh key from settings

This PR changes travis build to get `ssh_key` from payload, not from job's config. That way we can decide which key to send in travis hub. Currently I don't use `source`, because we can't distinguish between a key from a config file and a key from the DB.
So, basically I unified returning `source_key`, so it's all in one place and logic is handled on the backend, here is how `ssh_key` is serialized: https://github.com/travis-ci/travis-core/blob/master/lib/travis/api/v0/worker/job/test.rb#L69-L71 and here is how `job.ssh_key` works: https://github.com/travis-ci/travis-core/blob/master/lib/travis/model/job.rb#L98-L100

I will also need to change one more thing in that regard - currently we can't distinguish between `source_key` from `.travis.yml` and ssh key from repository.