Prevent CouchDB from starting on boot

It is hypothesized that the change of PPA to the official one (https://github.com/travis-ci/travis-cookbooks/commit/e622123bfef4b88cff8457e15a3e599033013512) switched the init script to Upstart, which apparently does not respect Chef's `disable` action.

This will prevent CouchDB from starting on boot (and consuming ~250 MB of memory).

This requires all images to be re-provisioned. Until then, we should hack travis-build to shut down couchdb at the start of the build.