Simplify releasing in CI

[Croydon]

Right now we run the entire CI jobs trough once, if successful so far we create a git tag, then running through everything again to get pretty much the very same results. This could be simplified by:

• first job stays our CI-prebuilds check/trigger
• second stage is our build + unit test step
  • with the difference that when we are in the master branch + no PR -> we use Travis Cache to store the built packages
• at the last stage, if every job before was successful AND we want to have a new tag/release, lets create a new tag, than instantly upload the binaries
• to not run every job again triggered by the new tag pushing

[Croydon]

CI build time is now smaller than it used to be; Flex has now a similar CI releasing strategy; the ideas from this issue would increase the complexity of your CI system even further.

=> Closing this because of irrelevance and (likely) unneccessary complexity