Add elasticsearch repository
Add elasticsearch repository

When writing a plugin for elasticsearch, it could potentially depend on Shield or any other library which is only released in [elasticsearch releases repository](http://maven.elasticsearch.org/releases).

As explained in [Shield documentation](http://www.elastic.co/guide/en/shield/current/_java_clients.html), Java users might need to add `elasticsearch-releases` to their repositories in `pom.xml`:

```xml
     <repository>
         <id>elasticsearch-releases</id>
         <url>http://maven.elasticsearch.org/releases</url>
         <releases>
            <enabled>true</enabled>
         </releases>
         <snapshots>
            <enabled>false</enabled>
         </snapshots>
      </repository>
```

Adding this repository in parent project would make that even easier for projects depending on elasticsearch-parent.

Closes #38.

elastic/elasticsearch-parent,dadoonet/elasticsearch-parent,rmuir/elasticsearch-parent