ubuntu 14.04 & optimization for smaller image size

Tested with success :
```
docker build -t mumble .
docker run -d -p 0.0.0.0:64738:64738 mumble
```

```
joseph$ docker logs 4dab93eb82f4e60cae9755aa58031d275fb15b939a1076c1ecdfb88ddf3bdf1d
<W>2014-08-17 20:41:11.679 Initializing settings from /etc/mumble-server.ini (basepath /etc)
<W>2014-08-17 20:41:11.680 OpenSSL: OpenSSL 1.0.1f 6 Jan 2014
<W>2014-08-17 20:41:11.742 ServerDB: Opened SQLite database /data/mumble-server.sqlite
<W>2014-08-17 20:41:11.743 Generating new tables...
<F>2014-08-17 20:41:11.758 Superuser password set on server 1

# ------------------------------------------------------------------------------
# SuperUser Password
#
# Password: Thophai9reih3bi
#
# Connect to the server as SuperUser and setup an admin user.
# ------------------------------------------------------------------------------

<W>2014-08-17 20:41:14.782 Initializing settings from /etc/mumble-server.ini (basepath /etc)
<W>2014-08-17 20:41:14.783 OpenSSL: OpenSSL 1.0.1f 6 Jan 2014
<W>2014-08-17 20:41:14.840 ServerDB: Opened SQLite database /data/mumble-server.sqlite
<W>2014-08-17 20:41:14.886 Failed to connect to D-Bus system
<W>2014-08-17 20:41:14.889 MurmurIce: Endpoint "tcp -h 127.0.0.1 -p 6502" running
<W>2014-08-17 20:41:15.104 Murmur 1.2.4 (1.2.4-0.2ubuntu1.1) running on X11: Ubuntu 14.04.1 LTS: Booting servers
<W>2014-08-17 20:41:15.113 1 => Server listening on [::]:64738
<W>2014-08-17 20:41:15.118 1 => Generating new server certificate.
<W>2014-08-17 20:41:15.360 1 => Announcing server via bonjour
<W>2014-08-17 20:41:15.364 1 => Not registering server as public
<W>2014-08-17 20:41:15.367 Object::connect: No such slot MurmurDBus::userTextMessage(const User *, const TextMessage &)
<W>2014-08-17 20:41:57.143 1 => <3:(-1)> New connection: 192.168.59.3:60060
<W>2014-08-17 20:41:57.213 1 => <3:(-1)> Client version 1.2.8 (OSX: 1.2.8)
<W>2014-08-17 20:41:57.218 1 => Starting voice thread
<W>2014-08-17 20:41:57.222 1 => CELT codec switch ffffffff80000010 0 (prefer ffffffff80000010) (Opus 1)
<W>2014-08-17 20:41:57.231 1 => <3:SuperUser(0)> Authenticated
```