Fix #base_image
Update image_creation.rb from ha-feature-trusty branch

As of https://github.com/travis-ci/travis-images/commit/c7bd96dca3f9c0c6d5a83f52976d0a67711136b5
Rather than cherry-picking the commits, simply copy the file from
that branch
Merge branch 'master' into ha-feature-trusty