Don't run apt-get update before each dependency

There's no need `apt-get update` before installing if we're just using
`state=installed`.

Removing these significantly speeds up the entire process.
Add 1 day cache timeout for Jenkins package

We want to the latest Jenkins (`state=latest`) so we do `apt-get update`
before attempting to install. It's overkill to do this more than once
per day, however, so set a cache time of 86,400 seconds.

(Also fix parameter name from `update-cache` to `update_cache` as the
the docs)
Ensure we apt-get update after adding Jenkins repo

Although it's the default behaviour, it's good to make sure we actually
do an `apt-get update` after adding the new Jenkins repository.

Also remove some dead (commented) code.
Merge pull request #12 from sealevelresearch/speed-up-provision

Speed up provision: be more selective about `apt-get update`