zmacs: don't require removed zile.registers module.

Forgot to remove these in changeset e6e89ee.
* bin/zmacs: Remove `require "zile.registers"`.
* bin/zz: Likewise.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>
travis: merge improvements from slingshot travis.yml.in.

* travis.yml.in: Merge improvements from slingshot, and simplify.
* .travis.yml: Regenerate.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>