Update go dep (#1560)

This fix updates go dep with `dep ensure --update` as well as the following:
- Removed github.com/ugorji/go restriction in Gopkg.toml (fixes  #1557)
- Added github.com/flynn/go-shlex in Makefile (neede by Caddy, maybe removed later)

This fix fixes #1557

Signed-off-by: Yong Tang <yong.tang.github@outlook.com>

vendor/google.golang.org/grpc/credentials/credentials.go

@@ -43,8 +43,9 @@ type PerRPCCredentials interface {
 	// GetRequestMetadata gets the current request metadata, refreshing
 	// tokens if required. This should be called by the transport layer on
 	// each request, and the data should be populated in headers or other
-	// context. uri is the URI of the entry point for the request. When
-	// supported by the underlying implementation, ctx can be used for
+	// context. If a status code is returned, it will be used as the status
+	// for the RPC. uri is the URI of the entry point for the request.
+	// When supported by the underlying implementation, ctx can be used for
 	// timeout and cancellation.
 	// TODO(zhaoq): Define the set of the qualified keys instead of leaving
 	// it as an arbitrary string.