Improve regexp to prevent false positives

If a filename happened to contain "db" and enough "rwx" characters before, then
this test would previously fail. For example:

```
drwxr-xr-x gitlab-runner/gitlab-runner      0 2015-04-02 07:46 uploads/tmp/cassidy.stokes8477/gitlabhq/36d972fa55d6b44810fc6fd843473adb/
```

Adding a space before the "db" match string tightens up the regexp and reduces the
chance of unintended match.

spec/tasks/gitlab/backup_rake_spec.rb

@@ -87,7 +87,7 @@ describe 'gitlab:app namespace rake task' do
       expect(tar_contents).to match('db/')
       expect(tar_contents).to match('uploads/')
       expect(tar_contents).to match('repositories/')
-      expect(tar_contents).not_to match(/^.{4,9}[rwx].*(db|uploads|repositories)\/$/)
+      expect(tar_contents).not_to match(/^.{4,9}[rwx].* (db|uploads|repositories)\/$/)
     end
 
     it 'should delete temp directories' do