http://askubuntu.com/questions/624701/where-are-cron-logs-stored-on-ubuntu-14-04

As default, cron's logs saved in /var/log/syslog. It depends on rsyslogd configuration. You can change it:

Go to rsyslog config

cd /etc/rsyslog.d/

sudo nano 50-default.conf

Uncoment line:

#cron.*                         /var/log/cron.log

Save file and restart rsyslog

sudo service rsyslog restart 

Restart your cron daemon for get it's messages from new file

sudo service cron restart