Fix Circle auth check for Terminus

[danielbachhuber]

The following no longer works because Terminus now returns 0 for logged out too:

terminus whoami > /dev/null
if [ $? -ne 0 ]; then
    echo "Terminus unauthenticated; assuming unauthenticated build"
    exit 0
fi

@greg-1-anderson @stevector Suggestions on what we might be able to use instead?

From https://github.com/lcache/wp-lcache/pull/115#issuecomment-297185829