default-jre-headless dependency for deb package (#89)

OpenDJ doesn't need Java GUI in some (most?) cases, but still requires full jre as must have dependency. 

Right now it can be tricked by installing opendj with following commands:
```
apt-get install default-jre-headless -y
dpkg -i --ignore-depends=default-jre opendj_4.4.3-1_all.deb
```

Everything works pretty fine so far except getting annoying message every attempt to install apt package:

```
root@localhost:~# apt-get install some-other-package
Reading package lists... Done
Building dependency tree       
Reading state information... Done
You might want to run 'apt --fix-broken install' to correct these.
The following packages have unmet dependencies:
 opendj : Depends: default-jre but it is not going to be installed or
                   java6-runtime or
                   java7-runtime
```
 

This patch alteres dependency in `opendj_X.Y.Z_all.deb` from `default-jre` to `default-jre-headless | default-jre` which solves the issue.