RFR: 241: Gradle version plugin should allow a version.txt file

[Erik Helin]

Hi all,

please review this small patch that makes the `VersionPlugin` look for a
`version.txt` in the root directory of the repository _if_ the `.git` directory
is missing. This is useful to be able to build Skara with a proper version from
e.g. a source tarball.

Thanks,
Erik

## Testing
- [x] Manual testing in directory without `.git` present
- [x] Manual testing in directory with `.git` present

-------------

Commits:
 - 5d866066: Look for version.txt if not a repository

Changes: https://git.openjdk.java.net/skara/pull/375/files
 Webrev: https://webrevs.openjdk.java.net/skara/375/webrev.00
  Issue: https://bugs.openjdk.java.net/browse/SKARA-241
  Stats: 15 lines in 1 file changed: 14 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/skara/pull/375.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/375/head:pull/375

PR: https://git.openjdk.java.net/skara/pull/375