Revert "Set correct default app.key configuration setting"

Summary:
This reverts commit 53de6354a0b38b229c091ff1fdf44f48eff7de7e.

T690 shows a case where it would be very welcome the application
loudly fails with an encryption key issue exception.

Test Plan:
Ensure CI tools (Jenkins jobs, insight.sensiolabs.com)
can create a .env file.

Reviewers: dereckson

Subscribers: alken-orin, Kaliiixx

Differential Revision: http://devcentral.nasqueron.org/D253