title: How do I fix cargo's "failed to parse manifest at ..." "editions are unstable" error?
body: <p>I tried to augment my rust project with a dependency on jni="0.12.3" and my next <code>cargo build</code> failed with the following error:</p>
  
  <pre><code>error: unable to get packages from source
  
  Caused by:
    failed to parse manifest at `/home/thoth/.cargo/registry/src/github.com-1ecc6299db9ec823/backtrace-0.3.32/Cargo.toml`
  
  Caused by:
    editions are unstable
  
  Caused by:
    feature `edition` is required
  
  consider adding `cargo-features = ["edition"]` to the manifest
  </code></pre>
  
  <p>I'm running cargo built from gentoo's dev-util/cargo-0.30.0 ebuild.</p>