Add a cronjob to dump DB daily

Summary:
We can then setup Tarsnap to backup these dumps elsewhere. These dumps are
currently only about 15 MB zipped.

Test Plan:
Ran `( cd $HOME/dumps && rethinkdb dump )` in $HOME on the server. Ensured dump
was found in the right folder.

Reviewers: xymostech, spicyj

Reviewed By: spicyj

Differential Revision: http://phabricator.benalpert.com/D211