flickr.rb: fix "[ERROR] flickr.rb: failed to get photo."

Do not read zero-byte cache, and delete cache if SSL certificate
verify failed.
js/flickr.js: use https for flickr api
Merge pull request #85 from minimum2scp/fix/flickr-ssl

flickr.rb creates zero byte cache and cause "[ERROR] flickr.rb: failed to get photo." without root certificates