We now build a docker base container based on debian sid (where the very
latest packages are available). That base container is updated once a
month, or whenever travis-build.Dockerfile or debian/control change, but
re-used for subsequent travis runs. While the initial build might take
up to 15 minutes, subsequent builds typically run in a minute or two.
All the different steps that we run on travis are now factored into
separate scripts in the travis/ directory.
Switching to docker should also help with issue #2174.
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