vendor: update all dependencies

* Update all dependencies
  * Remove all `[[constraint]]` from Gopkg.toml
  * Add in the minimum number of `[[override]]` to build
  * Remove go get of github.com/inconshreveable/mousetrap as it is vendored
  * Update docs with new policy on constraints

vendor/github.com/stretchr/testify/require/require.go.tmpl

@@ -1,6 +1,5 @@
 {{.Comment}}
 func {{.DocInfo.Name}}(t TestingT, {{.Params}}) {
-	if h, ok := t.(tHelper); ok { h.Helper() }
 	if !assert.{{.DocInfo.Name}}(t, {{.ForwardedParams}}) {
 		t.FailNow()
 	}