Change CDN links in Readme example

As Rawgit is closing example links will stop to function at the end of October 2019. I've chosen jsdelivr over unpkg as it's using GitHub as a source like Rawgit, rather than NPM, and have a bigger [market share](https://w3techs.com/technologies/comparison/cd-jsdelivr,cd-unpkg). Related issue #18.

README.md

@@ -134,12 +134,12 @@ Swift:
 
 ```html
 <!-- HTML -->
-<link rel="stylesheet" href="https://cdn.rawgit.com/tonsky/FiraCode/1.206/distr/fira_code.css">
+<link rel="stylesheet" href="https://cdn.jsdelivr.net/gh/tonsky/FiraCode@1.206/distr/fira_code.css">
 ```
 
 ```css
 /* CSS */
-@import url(https://cdn.rawgit.com/tonsky/FiraCode/1.206/distr/fira_code.css);
+@import url(https://cdn.jsdelivr.net/gh/tonsky/FiraCode@1.206/distr/fira_code.css);
 ```
 
 - IE 10+, Edge: enable with `font-feature-settings: "calt" 1;`