ci: Test go 1.15 instead of 1.12
ci: Test go 1.15 instead of 1.12

Testing the latest version should help catch problems due to changes in
Go.

snowballstem/snowball,snowballstem/snowball,snowballstem/snowball,snowballstem/snowball,snowballstem/snowball,snowballstem/snowball,snowballstem/snowball,snowballstem/snowball