og:image meta tag doesn't carry the scheme

[usamah82]

Most probably a quick fix, but I didn't have time to create a PR.

Basically at:

https://github.com/opoloo/lines-engine/blob/b1fcdcff33dd03a9b3835039c6e9939751a986c3/app/controllers/lines/articles_controller.rb#L59

meta_tags[:image] = CONFIG[:host] + @article.image_url if @article.image_url.present?

causes the og:image meta tag to not carry the scheme (http vs https), making FB scraper complain of mixed scheme / content issue.