Automate releases

[StefMa]

Currently I have to manually update the version and run the gradle task bintrayUpload.

We should automate this behaviour... somehow.
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One solution could be that we create a release branch.
And each time we merge into this branch the CI get triggered and publish it to bintray.
But that feels wrong.. somehow..
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Another solution would be that the CI get triggered each time we publish a tag.
Then the CI build this tag and publish it to bintray...