Switch documentation site to HTTPS

Accessing the website through HTTP may be blocked in some regions of the world.
Using HTTPS should help with this problem and is the good thing to do anyway.

docs/_config.yml

@@ -10,7 +10,7 @@ title: Spout
 email: oss@box.com
 description: "An open source PHP library to read and write spreadsheet files (XLSX, ODS and CSV), in a fast and scalable way."
 baseurl: "" # the subpath of your site, e.g. /blog
-url: "http://opensource.box.com" # the base hostname & protocol for your site
+url: "https://opensource.box.com" # the base hostname & protocol for your site
 
 # Build settings
 markdown: kramdown