Shared runners: increase `vm.max_map_count`

Per https://gitlab.com/gitlab-org/gitlab-ee/issues/2277 , the elasticsearch docker image is deprecated, and we may need to move to the official alternative in future.

However, this image requires the vm.max_map_count sysctl to be increased. I don't believe we can do that within .gitlab-ci.yml for a service container.

Here's the error message given if we try to use the new official images at present:

2017-04-27T17:38:57.519446405Z max virtual memory areas vm.max_map_count [65530] is too low, increase to at least [262144]