Make sure `docpad deploy-sunny` doesn't run on travis when no credentials

Looks like there's a failure in PR travis runs:
https://travis-ci.org/myplanetdigital/neompd/builds/13881134#L1393

It's not failing anything (which it probably should be... imho), but we can at least make it go away by testing if the secure credentials are available first:
http://about.travis-ci.org/docs/user/ci-environment/#Environment-variables

So test if `TRAVIS_SECURE_ENV_VARS` is there, and run deploy if present, but just `docpad generate` if not.