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/aws/aws-sdk-go/service/cloudsearchdomain/service.go

@@ -50,14 +50,14 @@ func New(p client.ConfigProvider, cfgs ...*aws.Config) *CloudSearchDomain {
 	} else {
 		c = p.ClientConfig(EndpointsID, cfgs...)
 	}
+	if c.SigningNameDerived || len(c.SigningName) == 0 {
+		c.SigningName = "cloudsearch"
+	}
 	return newClient(*c.Config, c.Handlers, c.Endpoint, c.SigningRegion, c.SigningName)
 }
 
 // newClient creates, initializes and returns a new service client instance.
 func newClient(cfg aws.Config, handlers request.Handlers, endpoint, signingRegion, signingName string) *CloudSearchDomain {
-	if len(signingName) == 0 {
-		signingName = "cloudsearch"
-	}
 	svc := &CloudSearchDomain{
 		Client: client.New(
 			cfg,