[Fix] Startpage ValueError on Spanish date format

datetime.parser.parse() does not know the Spanish date format which
leads to a ValueError. Fixes #1870

Traceback (most recent call last):
  File "/usr/local/searx/searx/search.py", line 160, in search_one_http_request_safe
    search_results = search_one_http_request(engine, query, request_params)
  File "/usr/local/searx/searx/search.py", line 97, in search_one_http_request
    return engine.response(response)
  File "/usr/local/searx/searx/engines/startpage.py", line 102, in response
    published_date = parser.parse(date_string, dayfirst=True)
  File "/usr/local/searx/searx-ve/lib/python3.6/site-packages/dateutil/parser/_parser.py", line 1358, in parse
    return DEFAULTPARSER.parse(timestr, **kwargs)
  File "/usr/local/searx/searx-ve/lib/python3.6/site-packages/dateutil/parser/_parser.py", line 649, in parse
    raise ValueError("Unknown string format:", timestr)
ValueError: ('Unknown string format:', '24 Ene 2013')

searx/engines/startpage.py

@@ -99,11 +99,14 @@ def response(resp):
         if re.match(r"^([1-9]|[1-2][0-9]|3[0-1]) [A-Z][a-z]{2} [0-9]{4} \.\.\. ", content):
             date_pos = content.find('...') + 4
             date_string = content[0:date_pos - 5]
-            published_date = parser.parse(date_string, dayfirst=True)
-
             # fix content string
             content = content[date_pos:]
 
+            try:
+                published_date = parser.parse(date_string, dayfirst=True)
+            except ValueError:
+                pass
+
         # check if search result starts with something like: "5 days ago ... "
         elif re.match(r"^[0-9]+ days? ago \.\.\. ", content):
             date_pos = content.find('...') + 4