Support for Cargo.lock format version 4

[joamag]

I'm getting compilation errors in Rust nightly due to the built's lack of support for the new Cargo.lock version 4.

Caused by:
  process didn't exit successfully: `/.../build/boytacean-d1602522edb07f70/build-script-build` (exit status: 101)
  --- stderr
  thread 'main' panicked at /root/.cargo/registry/src/index.crates.io-6f17d22bba15001f/built-0.7.4/src/dependencies.rs:151:59:
  Failed to parse lockfile: Parse("parse error: invalid Cargo.lock format version: `4`\n")
  note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

[lukaslueg]

Thanks for the heads-up. This is due to rust-lang/cargo#14595. We use cargo-audit's parser for Cargo.lock-files, the switch to v4 is tracked in rustsec/rustsec#1249.

[tarcieri]

Open PR here. Feedback appreciated: rustsec/rustsec#1206