travis: add "make uninstall" test

"make uninstall" is supposed to remove all the files that
"make install" (with the same arguments) have created.
This is a test to check that.

PS ideally, "make uninstall" should also remove any empty directories,
but let's not care about it for now.

Signed-off-by: Kir Kolyshkin <kir@openvz.org>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>