*: use opencontainers/go-digest package

The `digest` data type, used throughout docker for image verification
and identity, has been broken out into `opencontainers/go-digest`. This
PR updates the dependencies and moves uses over to the new type.

Signed-off-by: Stephen J Day <stephen.day@docker.com>

compose/schema/bindata.go

@@ -182,6 +182,7 @@ type bintree struct {
 	Func     func() (*asset, error)
 	Children map[string]*bintree
 }
+
 var _bintree = &bintree{nil, map[string]*bintree{
 	"data": &bintree{nil, map[string]*bintree{
 		"config_schema_v3.0.json": &bintree{dataConfig_schema_v30Json, map[string]*bintree{}},
@@ -234,4 +235,3 @@ func _filePath(dir, name string) string {
 	cannonicalName := strings.Replace(name, "\\", "/", -1)
 	return filepath.Join(append([]string{dir}, strings.Split(cannonicalName, "/")...)...)
 }
-