.pullapprove.yml: Reset on push, ignore authors, and require sign-offs

The sign-off requirement catches us up with fcc7f42 (Add contributing
and maintainer guidelines, 2016-05-03, opencontainers#1).  The author ignore catches
us up with c82a2e7 (MAINTAINERS: disallow self-LGTMs, 2016-05-27,
opencontainers#13).

The push reset catches us up with opencontainers/runtime-spec@aa9f3a26
(Add PullApprove checks, 2016-05-26, opencontainers/runtime-spec#458),
opencontainers/image-spec@95a46754d (Add PullApprove support to
enforce review, 2016-06-01, opencontainers/image-spec#101) and
opencontainers/runc@e2fd7c11 (Add PullApprove support, 2016-05-26,
opencontainers/runc#847).

Signed-off-by: W. Trevor King <[email protected]>

Author: wking

.pullapprove.yml

@@ -1,7 +1,10 @@
 approve_by_comment: true
 approve_regex: ^LGTM
 reject_regex: ^Rejected
-reset_on_push: false
+reset_on_push: true
+author_approval: ignored
+signed_off_by:
+  required: true
 reviewers:
   teams:
   - tdc-maintainers