[CI] Temporarily disable Travis for "master", use "0.4" branch
[API] Added a `clear_scroll` method

Specs: https://github.com/elasticsearch/elasticsearch/blob/master/rest-api-spec/api/clear_scroll.json
See:   http://www.elasticsearch.org/guide/en/elasticsearch/reference/current/search-request-search-type.html#clear-scroll

Closes #32