Fail Travis builds if Jekyll outputs empty files

Author: ashawley

.travis.yml

@@ -19,3 +19,5 @@ script:
 - bundle exec jekyll build 2> error.log
 - cat >&2 error.log
 - ( ! grep -qie Error error.log )
+- find ./_site -empty ! -name error.log
+- find ./_site -empty ! -name error.log | ! read