Fix Jekyll exit status for Travis

Grep the output of Jekyll for errors since it returns zero exits with
YAML errors.  See jekyll issue 5257.

Author: ashawley

.travis.yml

@@ -16,4 +16,6 @@ install:
 - bundle install
 
 script:
-- bundle exec jekyll build
+- bundle exec jekyll build 2> error.log
+- cat >&2 error.log
+- ( ! grep -qie Error error.log )