Change effective on 04 May 2018
Go version 1.10.2 is now the default and is used when either no version is specified or
go1.10 is specified.
Go version 1.9.6 is now used when
go1.9 is specified.
You can read more about the changes in these go versions here.
See the Heroku Devcenter article for more information on how to specify the Go version for the dependency manager you are using.