Install gearman from debian-packages

This will upgrade from `1.1.2` to `1.1.12`.

**Migration:**
The user `gearman` will be kept and re-used by the package.

```sh
monit unmonitor gearmand
rm /etc/init.d/gearman-job-server
rm -r /var/log/gearman-job-server/
rm -r /var/run/gearman-job-server/
rm /usr/local/sbin/gearmand

puppet agent -t
pkill -f '/usr/local/sbin/gearmand' # Stop old
/etc/init.d/gearman-job-server start # Start new
monit monitor gearman-job-server

monit restart php5-fpm
monit -g <app-name> restart
```

**Migration verified with:**
```sh
git checkout master
rake test:gearmand

git checkout issue-636
rake test:gearman:server keep_box=true # Fails

# --- doing above migration ---

rake test:gearman:server keep_box=true # Succeeds
```

Verified that CM's job-worker and client works with this gearman version.