Merge branch 'ss/travis-ci-force-vm-mode'

The "container" mode of TravisCI is going away.  Our .travis.yml
file is getting prepared for the transition.

* ss/travis-ci-force-vm-mode:
  travis-ci: no longer use containers

Author: gitster

.travis.yml

@@ -1,7 +1,5 @@
 language: c
 
-sudo: false
-
 cache:
   directories:
     - $HOME/travis-cache