Fixes error "Type str doesn't support the buffer API"

subprocess.check_output("mongod --version",shell=True) 
returns a byte string so to convert it to a regular string we should do this : 

mongo_version_string = subprocess.check_output("mongod --version",shell=True)
mongo_version_string = mongo_version_string.decode(encoding='UTF-8')

This gets rid of the "Type str doesn't support the buffer API"