remove deprecated user uploaded extension from .gitpod.yml

.gitpod.yml

@@ -4,4 +4,4 @@ tasks:
 
 vscode:
   extensions:
-    - rust-lang.rust@0.7.8:CvNqMTgDdt3UXt+6BCDTVg==
+    - rust-lang.rust@0.7.8

README.md

@@ -50,7 +50,7 @@ When you get a permission denied message then you have to exclude the directory
 
 [Run on Repl.it](https://repl.it/github/rust-lang/rustlings)
 
-[Open in Gitpod](https://gitpod.io/#https://github.com/rust-lang/rustlings)
+[![Open in Gitpod](https://gitpod.io/button/open-in-gitpod.svg)](https://gitpod.io/#https://github.com/rust-lang/rustlings)
 
 ## Manually