Avoid regexp warnings with Ruby < 1.9

Change regexp to avoid two warnings with older rubies.

As seen with the original regexp with ruby 1.8.7-p374 while running `rake spec`:
```
...
/tmp/specinfra/lib/specinfra/helper/detect_os/debian.rb:23: warning: regexp has invalid interval
/tmp/specinfra/lib/specinfra/helper/detect_os/debian.rb:23: warning: regexp has `}' without escape
...
```