Use a vendor directory for repeatable builds - fixes #816

This is using godep to manage the vendor directory.

vendor/github.com/aws/aws-sdk-go/service/sts/customizations.go

@@ -0,0 +1,12 @@
+package sts
+
+import "github.com/aws/aws-sdk-go/aws/request"
+
+func init() {
+	initRequest = func(r *request.Request) {
+		switch r.Operation.Name {
+		case opAssumeRoleWithSAML, opAssumeRoleWithWebIdentity:
+			r.Handlers.Sign.Clear() // these operations are unsigned
+		}
+	}
+}