Updates to core services in compose file

Changelog for 0.16.8-rc1:

• PR #304 Verify docker-compose images by @nitishkumar71

Commits
4d6b6df Update images for NATS / GW / queue-worker by @alexellis
b844a72 Bump images for nats, Prometheus and OpenFaaS CE by @alexellis
8227285 Update ISSUE_TEMPLATE.md by @alexellis
87773fd Verify docker-compose images by @nitishkumar71
ecf82ec Add a second link to the ROADMAP by @alexellis
9e8c680 Update README.md by @alexellis
85c1082 Add MaxIdleConns and MaxIdleConnsPerHost by @alexellis

Changes: 0.16.7...0.16.8-rc1

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