add github.com/docker/docker dependency

vendor/github.com/docker/go-connections/tlsconfig/certpool_other.go

@@ -0,0 +1,13 @@
+// +build !go1.7
+
+package tlsconfig
+
+import (
+	"crypto/x509"
+)
+
+// SystemCertPool returns an new empty cert pool,
+// accessing system cert pool is supported in go 1.7
+func SystemCertPool() (*x509.CertPool, error) {
+	return x509.NewCertPool(), nil
+}