Sphinx 1.10 and above want an explicit binlog_path

Not explicitly setting binlog_path causes it to fall back to the
compiled-in default (often the current directory), which may cause
searchd to not start:

    http://sphinxsearch.com/docs/current.html#conf-binlog-path

Add an explicit binlog_path, accompanied by a comment that is is only
valid on 1.10 and above, as some distributions only ship 0.9.8.