Migrate to dep (#3972)

* Update makefile to use dep

* Migrate to dep

* Fix some deps

* Try to find a better version for golang.org/x/net

* Try to find a better version for golang.org/x/oauth2

vendor/github.com/stretchr/testify/assert/assertion_forward.go.tmpl

@@ -1,4 +0,0 @@
-{{.CommentWithoutT "a"}}
-func (a *Assertions) {{.DocInfo.Name}}({{.Params}}) bool {
-	return {{.DocInfo.Name}}(a.t, {{.ForwardedParams}})
-}