Added _version_ field definition

Adding the `_version_` field definition allows a config.xml file generated with Haystack’s `build_solr_schema` command to work out-of-the-box with the “example” server in the Solr 4.2.0 distribution.

Omitting `_version_` will cause the stock “example” Solr server to start in an unusable state – for details, see:

* http://stackoverflow.com/a/15581292/298171
* http://lucene.472066.n3.nabble.com/Error-version-field-must-exist-in-schema-td4014352.html
* http://wiki.apache.org/solr/SolrCloud#Required_Config