travisify.sh: respect default branch name - #38

Nice! Here's a way to generalize the name of the remote beyond only origin:

upstreamBranch=$(git rev-parse --abbrev-ref --symbolic-full-name @{u})
remote=${upstreamBranch%/*}
defaultBranch=$(git remote show "$remote" | grep "HEAD branch" | sed 's/.*: //')

In other words: get the linked remote for the current branch, and assume that's the remote you care about for doing the release. Potential pitfalls:

1. If the current branch is local only, or not a branch. (The release script should fail in those cases anyway.)
2. If the current branch is main but of a fork. Then the tests will pass erroneously...

What do you think? I'm concerned about (2) above; maybe we should scrape the intended remote repository out of the pom.xml and then compare it to the list of remotes from git remote? It would be more work, but maybe more reliable.

If the current branch is main but of a fork. Then the tests will pass erroneously...

That's the case now already (if you're on master but of a fork), right? In the past this helped me setting up travis for repositories where I didn't have push permission (except the signing keys which failed IIRC), such as several repositories by @tischi where I made pull requests from my fork's master branch, so I wouldn't consider it an issue ;-)

@imagejan OK great, then can you quickly integrate and test my snippet above, and push a commit here if it's good? Then we can merge.

@ctrueden thanks! Committed, tested locally, and pushed. This pull request is now ready.