Dockerfile update

This PR updates the ```Dockerfile```:

* Ubuntu 14.04 is used
* redis, mysql and postgresql dependencies/startup were moved to an *optional* section
* Ruby version was updated
@dosire This was an upstart relevant workaround. I tried to reproduce the error with:

```bash
from ubuntu:14.04

run apt-get -y update
run sudo apt-get install atfrom ubuntu:14.04

run apt-get -y update
run sudo apt-get install at
```

No *initctl* error message will occur. Then I used the Ubuntu 14.04-upstart image as base image with:

```bash
from ubuntu-upstart:14.04

run apt-get -y update
run sudo apt-get install at
```

It shows:

```bash
initctl: Unable to connect to Upstart: Failed to connect to socket /com/ubuntu/upstart: Connection refused
```

When I add the ```dpkg-divert``` to the Ubuntu-upstart image:

```bash
from ubuntu-upstart:14.04

run apt-get -y update
run dpkg-divert --local --rename --add /sbin/initctl && rm -f /sbin/initctl && ln -s /bin/true /sbin/initctl

run sudo apt-get install at
```

No error messages are shown. 

That was the reason why I left out this fix!