Go apps using modules no longer need to specify a package spec to install.
Change effective on 09 April 2019
When Go applications that use modules are built by Heroku as part of
git push heroku master or via other applicable means, Heroku will detect, compile and install the main packages found in the codebase.
It should generally be okay to remove any
// +heroku install <package spec> comments in
go.mod files, unless the user’s intent is to limit compilation to a subset of packages.