Merge branch 'ls/travis-prove-order'

Merge branch 'ls/travis-prove-order'

Automated tests in Travis CI environment has been optimized by
persisting runtime statistics of previous "prove" run, executing
tests that take longer before other ones; this reduces the total
wallclock time.

* ls/travis-prove-order:
  travis-ci: explicity use container-based infrastructure
  travis-ci: run previously failed tests first, then slowest to fastest