rank_bm25 dependency is not optional

[kaaloo]

I get the following stack trace when running build_wikIR.py if the rank_bm25 dependency is not installed. Perhaps the README file for this project could be updated to remove indications as to the optional character of this dependency.

2021-01-14 1 wikiextractor/WikiExtractor.py:2429: DeprecationWarning: Flags not at the start of the expression '\\[(((?i)bitcoin:|ftp' (truncated)
   ExtLinkBracketedRegex = re.compile(
 wikiextractor/WikiExtractor.py:2436: DeprecationWarning: Flags not at the start of the expression '^(http://|https://)(' (truncated)
   EXT_IMAGE_REGEX = re.compile(
 wikiextractor/WikiExtractor.py:644: DeprecationWarning: The 'warn' function is deprecated, use 'warning' instead
   logging.warn("Template errors in article '%s' (%s): title(%d) recursion(%d, %d, %d)",
 WARNING: Template errors in article 'Prishtina' (222982): title(1) recursion(0, 0, 0)
 Traceback (most recent call last):
   File "build_wikIR.py", line 9, in <module>
     from rank_bm25 import BM25Okapi
 ModuleNotFoundError: No module named 'rank_bm25'

P.S. Thank you so much for your work on this ! 👍🏼