Use Azul Zulu JDK rather than Oracle

This is an upgrade from 

```
java version "1.7.0_09"
Java(TM) SE Runtime Environment (build 1.7.0_09-b05)
Java HotSpot(TM) 64-Bit Server VM (build 23.5-b02, mixed mode)
```

to

```
openjdk version "1.8.0_20"
OpenJDK Runtime Environment (Zulu 8.3.0.1-linux64) (build 1.8.0_20-b26)
OpenJDK 64-Bit Server VM (Zulu 8.3.0.1-linux64) (build 25.20-b23, mixed mode)
```

As a member of the technical community in GDS,
I would like to understand my options when it comes to choosing
technology
So that I can make the best choice for my context

Typically, that involves experience and data. I’ve volunteered us to try
the Azul OpenJDK port for maturity and stability in production.

Exciting, eh?
Merge pull request #630 from alphagov/feature/zulu

Use Azul Zulu JDK rather than Oracle