vendor: add missing deps and remove go get in .travis.yml

Signed-off-by: Akihiro Suda <suda.akihiro@lab.ntt.co.jp>

vendor/github.com/docker/docker/pkg/longpath/longpath.go

@@ -0,0 +1,26 @@
+// longpath introduces some constants and helper functions for handling long paths
+// in Windows, which are expected to be prepended with `\\?\` and followed by either
+// a drive letter, a UNC server\share, or a volume identifier.
+
+package longpath
+
+import (
+	"strings"
+)
+
+// Prefix is the longpath prefix for Windows file paths.
+const Prefix = `\\?\`
+
+// AddPrefix will add the Windows long path prefix to the path provided if
+// it does not already have it.
+func AddPrefix(path string) string {
+	if !strings.HasPrefix(path, Prefix) {
+		if strings.HasPrefix(path, `\\`) {
+			// This is a UNC path, so we need to add 'UNC' to the path as well.
+			path = Prefix + `UNC` + path[1:]
+		} else {
+			path = Prefix + path
+		}
+	}
+	return path
+}