circular-buffer: Use `[]` instead of `get().unwrap()`
circular-buffer: Use `[]` instead of `get().unwrap()`

Clippy suggested:
https://travis-ci.org/exercism/rust/builds/456356560
warning: called `.get().unwrap()` on a Vec. Using `[]` is more clear and more concise
   = help: for further information visit https://rust-lang-nursery.github.io/rust-clippy/v0.0.212/index.html#get_unwrap

petertseng/exercism-rust,exercism/xrust,petertseng/exercism-rust,petertseng/exercism-rust,exercism/xrust