Use openjdk7-jre rather than oracle java 7 jre

Hi,

We currently run Elasticsearch using openjdk at Mozilla and it runs fine.

Also, I had this error when docker tried to install oracle java (my docker host is a Ubuntu 13.10):

```
dpkg: dependency problems prevent configuration of oracle-java7-set-default:
 oracle-java7-set-default depends on oracle-java7-installer; however:
  Package oracle-java7-installer is not configured yet.

dpkg: error processing package oracle-java7-set-default (--configure):
 dependency problems - leaving unconfigured
Processing triggers for libc-bin (2.18-4) ...
Errors were encountered while processing:
 oracle-java7-installer
 oracle-java7-set-default
E: Sub-process /usr/bin/dpkg returned an error code (1)
```

This issue was resolved when using openjdk