Re: [PATCH v4 01/21] travis: fix skipping tagged releases

> From: Johannes Schindelin <johannes.schindelin@gmx.de>
>
> When building a PR, TRAVIS_BRANCH refers to the *target branch*.
> Therefore, if a PR targets `master`, and `master` happened to be tagged,
> we skipped the build by mistake.
>
> Fix this by using TRAVIS_PULL_REQUEST_BRANCH (i.e. the *source branch*)
> when available, falling back to TRAVIS_BRANCH (i.e. for CI builds, also
> known as "push builds").
>
> Let's give it a new variable name, too: CI_BRANCH (as it is different
> from TRAVIS_BRANCH). This also prepares for the upcoming patches which
> will make our ci/* code a bit more independent from Travis and open it
> to other CI systems (in particular to Azure Pipelines).