Pin tool versions using go modules

This pins down the versions of various tools used in the building,
testing, and linting of kustomize. This will provide more consistency
across contributors' and travis' environments.

tools/tools.go

@@ -0,0 +1,9 @@
+// +build tools
+
+package tools
+
+import (
+	// These imports are all tools used in the building and testing process
+	_ "github.com/golangci/golangci-lint/cmd/golangci-lint"
+	_ "github.com/monopole/mdrip"
+)