Process Fastly logs daily with cron job

The processing script needs rbenv and bundler, so prepend rbenv shims
to PATH in the environment setting.

Fastly logs are rotated at around 6:30am, but often as late as 6:50,
so processing them at 7:15 seems safe.
Merge pull request #127 from alphagov/automate_fastly_logs_processing

Process Fastly logs daily with cron job