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.

go.mod

@@ -7,15 +7,16 @@ require (
 	github.com/go-openapi/spec v0.19.2
 	github.com/gogo/protobuf v1.3.0 // indirect
 	github.com/golang/glog v0.0.0-20160126235308-23def4e6c14b // indirect
+	github.com/golangci/golangci-lint v1.17.1
 	github.com/gorilla/mux v1.7.3 // indirect
 	github.com/gorilla/sessions v1.2.0 // indirect
 	github.com/gorilla/websocket v1.4.1 // indirect
-	github.com/monopole/mdrip v0.2.48 // indirect
+	github.com/monopole/mdrip v0.2.48
 	github.com/pkg/errors v0.8.1
 	github.com/russross/blackfriday v2.0.0+incompatible // indirect
 	github.com/shurcooL/sanitized_anchor_name v1.0.0 // indirect
-	github.com/spf13/cobra v0.0.2
-	github.com/spf13/pflag v1.0.3
+	github.com/spf13/cobra v0.0.5
+	github.com/spf13/pflag v1.0.5
 	gopkg.in/yaml.v2 v2.2.2
 	k8s.io/api v0.0.0-20190313235455-40a48860b5ab
 	k8s.io/apimachinery v0.0.0-20190313205120-d7deff9243b1