Add CI integration and release builds

[menski]

• either use Zeebe Jenkins or Travis CI
• have a release process which creates a GitHub release with the release artifacts for the common platforms

Tasks

• Setup nightly builds
• Publish nightly to S3 bucket
• Publish releases to GitHub releases

[nikku]

Merging #33 made the Zeebe Modeler adopt the new automatic release procedure implemented in the Camunda Modeler.

It works as follows:

• A developer creates a release issue to track the release progress (based on the RELEASE_CHECKLIST)
• She triggers the release via npm run release
• The release script will ask the developer for the version bump (minor, major, patch)
• The release will be tagged and pushed
• CI takes over and builds the released tag (including code signing) (cf. .travis.yml for the actual magic)
• CI publishes the release to https://github.com/zeebe-io/zeebe-modeler/releases
• The published artifacts are in draft state and must manually (after integration testing) be published

I'll update the release procedures in a future PR.

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TLDR: @dominikhrn, @menski You may still trigger releases (if you want / need them). Just ensure you start it via the steps outlined above. We highly encourage you to create an issue containing the RELEASE_CHECKLIST to track the release progress. This allows you to offload work (such as integration testing on different platforms to us (@pinussilvestrus, @barmac, @philippfromme, @nikku).

[nikku]

The first nightly should build in the next 24h.

[nikku]

Closing issue, as all necessary improvements are in place.