Install repository key from repo, not HTTP

Jenkins unfortunately don't provide their repository key from an HTTPS
conneciton. Installing such a key without authentication is not good as
it is wide to to man-in-the-middle attacks.

This commit makes ansible install the key directly from the repository.
If the key changes, we will have to verify and update it here, but this
feels like a justifiable change.
Merge pull request #11 from sealevelresearch/install-apt-key-from-repo

Install repository key from repo, not HTTP