[build] Update to elasticsearch 2.0.0

We need to define `<elasticsearch.version>` in `pom.xml` as we now inherit from elasticsearch project parent pom.

But `elasticsearch.version` is set by default to `project.version` which is here `3.0.0-SNAPSHOT` for the mapper project.

Related to elastic#148

Author: dadoonet

pom.xml

@@ -33,6 +33,7 @@
 
     <properties>
         <!-- If we need to define any specific property -->
+        <elasticsearch.version>2.0.0-SNAPSHOT</elasticsearch.version>
     </properties>
 
     <dependencies>