switch to golangci-lint linter (#539)

.circleci/config.yml

.golangci.yml

.gometalinter.json

Makefile

loki-build-image/Dockerfile

pkg/chunkenc/gzip.go

pkg/distributor/distributor.go

pkg/ingester/chunk_test.go

pkg/ingester/client/client.go

pkg/ingester/flush_test.go

pkg/ingester/stream_test.go

pkg/iter/iterator_test.go

pkg/logproto/logproto.pb.go

pkg/promtail/promtail_test.go

pkg/promtail/targets/filetarget_test.go

pkg/querier/store.go