feat(post): add cargo demo gif

Author: ashleygwilliams

_posts/2018-10-25-Rust-1.30.md

@@ -352,7 +352,7 @@ See the [detailed release notes][notes] for more.
 The largest feature of Cargo in this release is that we now [have a progress
 bar!](https://github.com/rust-lang/cargo/pull/5995/)
 
-> GIF goes here
+![demo gif](/images/2018-10-25-Rust-1.30/demo.gif)
 
 See the [detailed release notes][notes] for more.