fixed dependencies

vendor/gonum.org/v1/gonum/mathext/doc.go

@@ -0,0 +1,7 @@
+// Copyright ©2017 The Gonum Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package mathext implements special math functions not implemented by the
+// Go standard library.
+package mathext // import "gonum.org/v1/gonum/mathext"