README.md should recommend https links instead of http

[Loksly]

Description

The README.md file suggests the usage of links like:

• http://naver.github.io/billboard.js/release/latest/dist/billboard.js
• http://naver.github.io/billboard.js/release/latest/dist/billboard.css

Suggested fix

It is supported and much better to use the https links:

• https://naver.github.io/billboard.js/release/latest/dist/billboard.js
• https://naver.github.io/billboard.js/release/latest/dist/billboard.css

Please consider to add an 's' on every link to github.io cdn.

[netil]

Hi @Loksly, thanks for suggesting.
Updated every links' protocol to https on readme file. 😃