cargo test warns with lint unused_doc_comment has been renamed

[AndyGauge]

warning: lint unused_doc_comment has been renamed to unused_doc_comments

  --> /tmp/rust-skeptic.Aq6OdV6EK5TS/test.rs:13:1
   |
13 | / error_chain! {
14 | |    foreign_links {
15 | |        CsvError(csv::Error);
16 | |        ParseInt(std::num::ParseIntError);
...  |
20 | |    }
21 | | }
   | |_^
   |

= note: this error originates in a macro outside of the current crate (in Nightly builds, run with -Z external-macro-backtrace for more i

[budziq]

Moving to error_chain = 0.12.0 should silence this error rust-lang-deprecated/error-chain#245