Revert ".travis.yml: omit linux-ppc64le target."

IBM POWER Open Source Ecosystem division asserts commitment to providing
more reliable service. GH#7016.

This reverts commit 275bfc56

.

Reviewed-by: Paul Dale <paul.dale@oracle.com>
Reviewed-by: Matt Caswell <matt@openssl.org>
Reviewed-by: Richard Levitte <levitte@openssl.org>