Flush stdout after a notification has been printed

Summary: Fixes T754.

Test Plan:
Fire a notification and see if it's correctly printed.

./notifications | tee -a /home/dereckson/public_html/notifications.log
[06:56:32] <Nasqueron/nasqueron> dereckson created T754: ./notifications > file.log or | tee file.log doesn't immediately flush messages.

Reviewers: dereckson

Subscribers: Kaliiixx, rama

Maniphest Tasks: T754

Differential Revision: http://devcentral.nasqueron.org/D291