Revert "revert node to 8.15.1"

This reverts commit bec63c1ad0.

ldap: unbind callback never fires in node 10.15 it seems. otherwise,
it all works..

src/test/backups-test.js

@@ -70,8 +70,6 @@ function cleanupBackups(callback) {
 }
 
 describe('backups', function () {
-    this.timeout(10000);
-
     before(function (done) {
         const BACKUP_DIR = path.join(os.tmpdir(), 'cloudron-backup-test');