CI: use a pipeline: first build the tarball, then check it

Build the tarball in one job, check it in many.
Unfortunately no real gain in overall duration.
With help from Clément Démoulins.

* .travis.yml: here.
Remove all the tricks that were used to be able to boostrap on old
distros.
(before_install): Merge into 'script', because before_install applies
to all the jobs, and we don't want to run it for the 'compile' job.

THANKS

@@ -33,6 +33,7 @@ Bruce Lilly               [email protected]
 Bruno Haible              [email protected]
 Charles-Henri de Boysson  [email protected]
 Christian Burger          [email protected]
+Clément Démoulins         [email protected]
 Colin Daley               [email protected]
 Cris Bailiff              [email protected]
 Cris van Pelt             [email protected]