rebase: remove "github.com/pkg/errors" from go.mod

There is no direct dependency on "github.com/pkg/errors" anymore, so it
can be removed from go.mod.

After running `go mod tidy`, the "github.com/pkg/errors" gets downgraded
to a version that is referenced in other depndencies. This was
unexpected, but seems needed.

Signed-off-by: Niels de Vos <ndevos@redhat.com>

vendor/github.com/pkg/errors/.travis.yml

@@ -1,10 +1,15 @@
 language: go
 go_import_path: github.com/pkg/errors
 go:
+  - 1.4.x
+  - 1.5.x
+  - 1.6.x
+  - 1.7.x
+  - 1.8.x
+  - 1.9.x
+  - 1.10.x
   - 1.11.x
-  - 1.12.x
-  - 1.13.x
   - tip
 
 script:
-  - make check
+  - go test -v ./...