Update elasticsearch dependency to 1.2.0
Add plugin version in es-plugin.properties

With https://github.com/elasticsearch/elasticsearch/issues/2784, we can
now add plugin version in `es-plugin.properties` file.

It will only be used with elasticsearch 1.0.0 and upper.
Enforce java version 1.7

Java 1.7 is mandatory to elasticsearch 1.2.x as it depends on Lucene 4.8