Use HTTPS for curl installation (#627)

Using HTTPS prevents: 

- accidentally running a captive portal login screen as a Fish script and sourcing it
- other security and privacy benefits of HTTPS

README.md

@@ -14,7 +14,7 @@ Looking for plugins? Browse [git.io/awsm.fish](https://git.io/awesome.fish) or [
 ## Installation
 
 ```console
-curl -sL git.io/fisher | source && fisher install jorgebucaran/fisher
+curl -sL https://git.io/fisher | source && fisher install jorgebucaran/fisher
 ```
 
 ## Quickstart
@@ -150,4 +150,4 @@ end
 
 ## License
 
-[MIT](LICENSE.md)
+[MIT](LICENSE.md)