Deal with github renaming the default branch from master to main

[tpietzsch]

For newly created repositories, github names the default branch "main" by default now.

travisify.sh, release-version.sh, and others(?) hvae to be updated to work on both "master" and "main"

[ctrueden]

The travisify script was fixed by @imagejan with #38. The release-version script was fixed by me with a51f894.

There are still a few places in other scripts where master is hardcoded, so I'll leave this open for now.

[ctrueden]

The only places where master are still hardcoded are git-hg-synchronizer and travis-build, both of which are legacy scripts kept only for backwards compatibility. I may delete them at some point, since no core SciJava workflows rely on them anymore.