Travis CI build template

[cskiraly]

This issue is to track variants of our Travis CI build template and to have a central place for discussions about the build process.

This is the current template: https://github.com/Agile-IoT/agile-osjs/blob/8c5c49ba536496609bcfa577f3df878f115fe06f/.travis.yml
Another variant from @craig-mulligan, adding versionist and other tweaks: https://github.com/Agile-IoT/agile-ui/blob/9155f01c81d1d8d6db1599eb7a331de9148e86a6/.travis.yml

Related issues:
Agile-IoT/agile-scripts#2

[cskiraly]

The newest template at https://github.com/Agile-IoT/agile-core/blob/79b449ca0890bc6627efd368891e3a51407a1f71/.travis.yml adds these features:

• building with docker 17.05.0-ce, with support for multi-stage builds
• tagging pull request based images with a special docker tag: $TRAVIS_BRANCH-pr-$TRAVIS_PULL_REQUEST

[cskiraly]

new version (eclipse-agail/agile-core@5642b4e) is pinning also the base image version for the x86_64 version.

[cskiraly]

Newest template based on https://github.com/Agile-IoT/agile-ci-scripts is at https://github.com/Agile-IoT/agile-nodered/blob/acb34e7e6374941ad46cced0de05f6b955a87c1a/.travis.yml