Replace godep with dep

vendor/github.com/opencontainers/go-digest/.pullapprove.yml

@@ -1,12 +0,0 @@
-approve_by_comment: true
-approve_regex: '^(Approved|lgtm|LGTM|:shipit:|:star:|:\+1:|:ship:)'
-reject_regex: ^Rejected
-reset_on_push: true
-author_approval: ignored
-signed_off_by:
-  required: true
-reviewers:
-  teams:
-  - go-digest-maintainers
-  name: default
-  required: 2