Fix for #178 and autodetect mongo version

[gummiboll]

Connection check always returns OK since MongoClient doesn't block since pymongo 3.0 (see: http://api.mongodb.org/python/current/api/pymongo/mongo_client.html). With this pull-request check_mongodb.py tries to determine which version of mongodb is running directly after mongo_connect so if a bogus host/port is used or mongodb isnt running it will throw a error.

-M is kept so it doesn't break current installations, if -M is specified mongo_version is set to what the user specified if not the autodetected version is used. Also changed mongo_version to a int instead of string.

PS. Don't have any mongodb 3.x installation to test this on so not certain it wont break those.
PPS. Also removed some trailing whitespaces.

[warrenpnz]

Nice, I was just removing the -M option and using # mongo_version = con.server_info()['version'].split('.')[0].strip()
Yours is a lot more error resistant, so I've updated mine to use that now. +1