Add SKYLIGHT_AUTHENTICATION env var in production

[sauloperez]

In order for coopdevs/timeoverflow#322 to work, we need to make the SKYLIGHT_AUTHENTICATION env var available in production with the appropriate value. More details in #322 .

[mperezv]

Solved here?

[sauloperez]

Yep. And the actual proof is https://github.com/coopdevs/timeoverflow-provisioning/blob/master/inventory/host_vars/timeoverflow.org/secrets.yml#L34