.travis.yml: Use bionic (Ubuntu 18), add many improvements
.travis.yml: Use bionic (Ubuntu 18), add many improvements

Update to bionic (Ubuntu 18) and openjdk10.

Also add other improvements, such as:

* Add mmverify.py for iset.mm.  It's slow, so I haven't added a
  job for it for set.mm, but this is a first step.
* Add a date validation job.  This uses the script
  scripts/report-changes.py to convert dates to Unix timestamps;
  as a side-effect, if it *can't* do that it will cause a failure.
  That will prevent future "31 June" incidents.
* Add as a *comment* an mm-scala verification job.
  The verification job is not working correctly, so it's a just
  a comment, but it should be helpful as a starting point.
* Include verifier developer names in the job names.
* Add caching directives.  They aren't helping much currently,
  but hopefully they will provide a starting point.

Signed-off-by: David A. Wheeler <9ae72d22d8b894b865a7a496af4fab6320e6abb2@dwheeler.com>

metamath/set.mm,metamath/set.mm,metamath/set.mm