Switch from Travis and AppVeyor to GitHub actions

[jbo-ads]

The changes proposed in this pull request aim at:

• Upgrading the CI platforms on which builds and basic tests are performed prior merging, from Ubuntu 12.04, 14.04 and 16.04 to Ubuntu 18.04 and 20.04.
• Switching CI providers from Travis and AppVeyor to GitHub Actions.

Vagrantfile also has been upgraded to Ubuntu 20.04.
Some minor bug found along the way have been fixed as well.

Comment are welcomed, in particular from Windows users.

[tbonfort]

I haven't looked at the workflow yaml itself, but all the rest looks good to me

[geographika]

Looks good, as does the output at https://github.com/jbo-ads/mapcache/actions/runs/1573166087 (plus the new test output).
Will Appveyor be kept around for a couple of months before depracting, running in tandem with GHA?
I note the switch to PowerShell - @rouault is a big fan!

[rouault]

I note the switch to PowerShell - @rouault is a big fan!

obviously everyone has understood that this was hopefully ironic :-) I'd have gone with Bash on Windows personally but Jerome did the work already

[jbo-ads]

@geographika - This pull request adds GitHub Actions without suppressing AppVeyor and Travis CI. We can let run both during a few months and pull the trigger when the new approach is validated.
Powershell is not a choice of mine (see the number of commits before reaching a good state...) @rouault : My first attempt was with bash, until I realized that handling windows filepaths stored in default variables such as ${{github.workspace}} would have been a biggest nightmare than learning Powershell basics. Si fueris Romae, Romano vivito more