travis: using old configuration with current scripts

[OlegHahm]

The discussion in #4389 (comment) revealed that we have problem with older PRs if the Travis configuration and/or the test scripts change. One possible solution that I see, could be by parameterizing the build_and_test script with the branch name of the current build and let Travis run the version of the script that belongs to the currently checked PR.

[miri64]

The problem why this arises is this rebase. Travis usually creates a merge commit with the current merge base on build creation (which means it doesn't change if the build is just restarted) so this rebase is most likely only there to catch errors that happen due to changes in master after build creation.

[miri64]

Removing this rebase would result in exactly the behavior you proposed on the expense of breaking master due to errors that the version of s PR just wouldn't pick up because it uses an old version of master.

[miri64]

I think I have an idea how to fix that, but first I go watch Deadpool now ;)

[miri64]

The fix is now available at #4921 btw ;-)