pkg/errors: use a lower bound (#1238)

Other projects are now depending directly on PD.

In the long term we need to separate out client/library functionality
from the server. But that will still require this change.

Gopkg.toml

@@ -38,7 +38,7 @@
 
 [[constraint]]
   name = "github.com/pkg/errors"
-  version = "0.8.0"
+  version = ">= 0.8.0"
 
 [[override]]
   name = "github.com/BurntSushi/toml"