(maint) Upgrade TravisCI docker version
(maint) Upgrade TravisCI docker version

 - This repo requires Docker buildkit to build containers in CI, which
   enables skipping optional stages in multi-stage builds (which skips
   building packages when not creating edge containers). Standard
   `docker build` does not support this, nor does it handle setting ARG
   valeus across stages as this Dockerfile needs.

 - Travis has been failing frequently with an error like:

   failed to solve with frontend dockerfile.v0: failed to build LLB:
   failed commit on ref "layer-sha256:XXX":
   unexpected commit size 0, expected 162: failed precondition

   This appears to be related to the 19.03 version of Docker / Buildkit
   in use in Travis as this problem only shows up there.

   This install adds about 30s to the cell execution time.

puppetlabs/puppet-server,puppetlabs/puppetserver,puppetlabs/puppet-server,puppetlabs/puppetserver,puppetlabs/puppetserver,puppetlabs/puppetserver,puppetlabs/puppet-server