Golang 1.15 comes to few improvements, one of them is to have smaller
binary.
This PR is to make golang 1.15 as default version in CI, I also update
Docker based image to golang:1.15* as well.
Two issues faced with golang 1.15:
- Conflict between -v in `golangci-lint` and `go test`. Update to --verbose
to avoid the same. [1]
- `nolintlint_unused.go` testdata is not matching regex. Correct by adding one
space after //
[1]: https://github.com/golang/go/issues/40763
Signed-off-by: Tam Mach <sayboras@yahoo.com>
