CI and Node.js updates - Githubissues

Several changes regarding CI and Node.js versions

Docker builds are run off of our central ci-scripts
Support for Node.js 6 has been dropped (https://github.com/pelias/pelias/issues/752)
Semantic-release was running (doing nothing but taking up build time) on all pull requests for no reason
Readme is updated to point to a central version requirement document rather than listing the supported Node.js version
Releases now run off the master branch (https://github.com/pelias/pelias/issues/749)

pelias / pip-service