Reduce expected size of riak database backup

```
Failures:

  1) Database::Riak With Compression
     Failure/Error: expect( job.package ).to match_manifest(%q[
       expected that:

       drwxrwxr-x vagrant/vagrant   0 2013-12-03 15:37 my_backup/
       drwxrwxr-x vagrant/vagrant   0 2013-12-03 15:37 my_backup/databases/
       -rw-rw-r-- vagrant/vagrant 6338 2013-12-03 15:37 my_backup/databases/Riak-riak@127.0.0.1.gz

       would match:

             6500..6700 my_backup/databases/Riak-riak@127.0.0.1.gz

     # ./spec/acceptance/database/riak_spec.rb:40:in `block (2 levels) in <module:Backup>'
```