Should skip deploy if code is from a fork

[philwinder]

If creating a PR from a fork, the deployment will fail in travis because it doesn't allow forks access to secret variables:

The command "openssl aes-256-cbc -K $encrypted_a10f349f39af_key -iv $encrypted_a10f349f39af_iv -in front-end_deploy_rsa.enc -out front-end_deploy_rsa -d" failed and exited with 1 during .

Skip deployment on forked PRs.

[abuehrle]

@philwinder just updating the flux guide where we tell people to fork the demo. Is there a workaround for this?