Allow for build matrices

[probonopd]

Allow for uploading the build artifacts of build matrices (e.g., multiple architectures). For this, we need to check the commit sha of the current build and check whether there is already a release for the same sha. If it is, then do not delete the release and the tag, but upload additional binaries to it instead.

[TheAssassin]

With the now available build stages on Travis (still a beta feature), this seems doable. You can add a build stage after your main matrix build, the build for it will only be triggered after all the other ones succeeded. The question is only, how to share artifacts with that final step.

[probonopd]

Interesting. 👍