don't run live test if API secret key is not defined

Travis doesn't export the secure data (API secret key) when
building pull requests (rightly so, as otherwise anyone could
use the secret key, they would just need to submit a pull request).

This means all the pull request builds fail, which is annoying and
easily hides actual errors.
Merge pull request #27 from senko/silence-livetest

Don't run live test if API secret key is not defined