Make headings compatible with Outline Mode
Use custom script to provision Travis
Fix Rubocop tests
Fix Go tests
Add test requirements to contribution guidelines
Remove custom test VM setup

We now use Travis CI again, so our VM becomes superfluous.
Pass sphinx flags to Makefile
Merge branch 'rework-travis-ci'

* rework-travis-ci:
  Pass sphinx flags to Makefile
  Remove custom test VM setup
  Add test requirements to contribution guidelines
  Fix Go tests
  Fix Rubocop tests
  Use custom script to provision Travis
[Fix #375] Guard use of package-user-dir

In some rare cases this variable might not be defined
Remove superfluous debugging output
Support checking for Rust test functions.

Without the --test flag, functions marked as #[test] are not checked for
errors.