vendoring: update cortex to latest master (#938)

* update cortex to latest master

using v3.4.0-rc.1 for go.etcd.io/etcd
v0.0.2 for github.com/prometheus/procfs
v1.0.0 for github.com/prometheus/client_golang
latest master for github.com/weaveworks/common with changes from https://github.com/weaveworks/common/pull/153

* fixed failing tests

* use large instance for test and lint jobs in CircleCI

* running only 6 test binaries in parallel

* removed resource type change for CircleCI

* changed GOGC to 10 for lint makefile target