travis-ci: Enable ia32 tests

Travis CI with Xenial has 4.15 kernel these days - all support for ia32
C/R should be inplace. Finally :)

Putting it into "allow_failures" to let it soak a bit.

Signed-off-by: Dmitry Safonov <dima@arista.com>
Signed-off-by: Andrei Vagin <avagin@gmail.com>