.travis.yml: run integration tests with BUILDAH_ISOLATION=chroot
.travis.yml: run integration tests with BUILDAH_ISOLATION=chroot

Run integration tests with both BUILDAH_ISOLATION=oci and
BUILDAH_ISOLATION=chroot.

Signed-off-by: Nalin Dahyabhai <nalin@redhat.com>

Closes: #836
Approved by: rhatdan

projectatomic/buildah,rhatdan/buildah,nalind/buildah-1,TomSweeneyRedHat/buildah,nalind/buildah-1,rhatdan/buildah,ipbabble/buildah,ipbabble/buildah,TomSweeneyRedHat/buildah,nalind/buildah,projectatomic/buildah,TomSweeneyRedHat/buildah,projectatomic/buildah,nalind/buildah,rhatdan/buildah,nalind/buildah-1