Lock the short commit IDs used to 7 characters

To prevent AppVeyor's usage of Git v2.11.0 from causing it to deploy
different Bintray versions from Travis, which still uses Git v1.8. Fixes
#744.

scripts/archive.js

@@ -14,6 +14,7 @@ function getGitDescription() {
     'describe',
     '--tags',
     '--long',
+    '--abbrev=7',
   ])).slice(0, -1);
   let branch = String(helpers.safeExecFileSync('git', [
     'rev-parse',