Update sphinxsearch to 2.1.5.

Changes since 2.1.4:
- fixed #1848, infixes and morphology clash
- fixed #1823, indextool fails to handle indexes with lemmatizer morphology
- fixed #1799, crash in queries to distributed indexes with GROUP BY on
  multiple values
- fixed #1718, expand_keywords option lost in disk chunks of RT indexes
- fixed documentation on rt_flush_period
- fixed network protocol issue which results in timeouts of libmysqlclient
  for big Sphinx responses