Update textproc/sphinxsearch to 2.2.8.

2.2.8
- It is now possible to add an option to an agent description.
- fixed #2182, incorrect query results with multiple same destination
  wordforms
- fixed #2181, improved error message on incorrect filters
- fixed #2178, ZONESPAN operator for queries with more than two words
- fixed #2172, incorrect results with field position fulltext operators
- fixed #2171, some index options do not work for template indexes
- fixed #2170, joined fields indexation with document id equals to 0
- fixed #2110, crash on snippet generation related to blend_chars inside
  HTML markup
- fixed WLCCS ranking factor computation
- fixed memory leak on queries with ZONEs

2.2.7
- added #2112, string equal comparison support for IF() function (for JSON
  and string attributes)
- added #2153, IN() support for mixed and top-level JSON array
- fixed #2158, crash at RT index after morphology changed to AOT after
  index was created
- fixed #2155, stopwords got missed on disk chunk save at RT index
- fixed #2151, agents statistics missed in case of huge amount of agents
- fixed #2139, escape all special characters in JSON result set, according
  to RFC 4627
- fixed #2123, no pid file created in x64 release built with vs2012
- fixed #2115, indexer crash on wordforms with multiple destination keywords
- fixed #2050, multi result set doesn't work without libmysqlclient
- fixed #2003, lemmatize_XX_all handling of short and exact words
- fixed #1912, reduce indextool memory usage during a check of a huge index
- fixed off by one errors in filtering of BIGINT attributes
- fixed seamless rotation in prefork mode
- fixed snippets crash with blend chars at the beginning of a string

(fhajny)