I already diagnosed this in https://mail.python.org/archives/list/python-dev@python.org/message/3Z4FNPEFTFTYDX6RYOQ54UKOVLQAWUEU/:

* https://travis-ci.org/python/cpython/jobs/616384134 looks for `pythonX.Y` but doesn't account for the fact that it could be Pyenv's stub
* https://travis-ci.org/python/cpython/builds/616384157 is trying to use Python version(s) that are not guaranteed to be preinstalled in the VM for all `language:`s.

I could fix this but I need to know what the hard requirements are to pick an optimal configuration.

* Is the (preinstalled only for `language: c`) `clang 7.0.0` required? Would 3.4-4.0 (available from xenial distro) do?
* Is `xenial` required? Would `bionic` do? In bionic, clang 7 is available from distro.
* Which Python version needs to be preinstalled? Tools/ssl/multissltests.py says it's supposed to be compatible with 2.7 and 3.4-3.7. Options are, by decreasing "sum of flexibility+reliability":
  * use 3.5.1 or 2.7.12 from distro
  * hard require `language:python; python: 3.7` (then we cannot use custom `clang`)
  * use another pyenv-preinstalled version (there are also 3.6 and 2.7 that are supposed to be preinstalled but there might be none)