word-count: use `str#to_lowercase`
word-count: use `str#to_lowercase`

This was added in Rust 1.2.0, making the custom `lowercase` function
unnecessary.

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