Update cortex vendoring (#325)

Gopkg.lock

Gopkg.toml

pkg/ingester/flush.go

pkg/ingester/ingester.go

...exproject/cortex/pkg/chunk/aws/dynamodb_storage_client.go

...github.com/cortexproject/cortex/pkg/chunk/aws/fixtures.go

....com/cortexproject/cortex/pkg/chunk/cassandra/fixtures.go

...ortexproject/cortex/pkg/chunk/cassandra/storage_client.go

.../github.com/cortexproject/cortex/pkg/chunk/chunk_store.go

...b.com/cortexproject/cortex/pkg/chunk/encoding/bigchunk.go

...thub.com/cortexproject/cortex/pkg/chunk/encoding/delta.go

...om/cortexproject/cortex/pkg/chunk/encoding/doubledelta.go

...ub.com/cortexproject/cortex/pkg/chunk/encoding/factory.go

...hub.com/cortexproject/cortex/pkg/chunk/encoding/varbit.go

...cortexproject/cortex/pkg/chunk/encoding/varbit_helpers.go

...rtexproject/cortex/pkg/chunk/gcp/bigtable_index_client.go

...texproject/cortex/pkg/chunk/gcp/bigtable_object_client.go

...github.com/cortexproject/cortex/pkg/chunk/gcp/fixtures.go

vendor/github.com/cortexproject/cortex/pkg/chunk/gcp/fnv.go

...m/cortexproject/cortex/pkg/chunk/gcp/gcs_object_client.go