ci: upgrade golangci-lint to v1.15.0

fixes unparam false-positives

Author: markus-wa

.travis.yml

@@ -27,7 +27,7 @@ before_script:
   - git fetch
 
   # Install golangci-lint
-  - curl -sfL https://install.goreleaser.com/github.com/golangci/golangci-lint.sh | sh -s -- -b $GOPATH/bin v1.12.2
+  - curl -sfL https://install.goreleaser.com/github.com/golangci/golangci-lint.sh | sh -s -- -b $GOPATH/bin v1.15.0
 
   # Download test data
   - pushd cs-demos && git lfs pull -I '*' && popd