Update to Go 1.15.1 for .tool-versions file
Now that https://gitlab.com/gitlab-org/security-products/license-management/-/merge_requests/214 is merged, this project that relies on it should be able to move forward to the latest stable Go version.
This will help folks building GDK with the latest Go version (per: gitlab-development-kit!1507 (merged)).
This is very much like !308 (merged).
Edited by Evan Read