Fix a clippy warning

[zzl0]

This diff fixes the clippy warning about manual_strip: https://rust-lang.github.io/rust-clippy/master/index.html#manual_strip

[markbt]

String::strip_prefix was added in 1.45 but our MSRV is 1.41. 1.45 is nearly 5 years old at this point, so perhaps we could consider bumping the MSRV. I think the reason for holding back was the version of Rust in Debian stable, so will need to check that.