Upgrade JGit version used by Gradle

[bluekeyes]

When trying to release 0.9.0, I saw the JGit issue where it thinks the repository is dirty, but running git status shows a clean state (and also fixes the state seen by JGit).

I believe this is fixed in newer versions of JGit, but I'm not sure if there's a newer release of the flexversion plugin. We might need to force upgrade JGit or switch to a different versioning method.