Add metadata endpoints (#1220)

See travis-ci/travis-ci#1220 for more info

This adds the necessary API endpoints for the metadata feature.

This PR is technically dependent on travis-ci/travis-core#233, but through the magic of Git and manual `Gemfile.lock`-editing I managed to get the Gemfile updated already. Unless more changes go into that PR, no update is needed for this, and should be mergeable.

## TODO

- [x] Add endpoint to fetch metadata for a job
- [x] Add endpoint to update metadata for a job