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/github.com/imdario/mergo/merge.go

@@ -54,7 +54,7 @@ func deepMerge(dst, src reflect.Value, visited map[uintptr]*visit, depth int, co
 		visited[h] = &visit{addr, typ, seen}
 	}
 
-	if config.transformers != nil {
+	if config.transformers != nil && !isEmptyValue(dst) {
 		if fn := config.transformers.Transformer(dst.Type()); fn != nil {
 			err = fn(dst, src)
 			return