Failed to run custom build command

Building proc-macro-error-attr v0.4.10 as a dependency fails on rust versions that don't report a build date:

# rustc --version
rustc 1.40.0

# cargo install cargo-udeps
...
error: failed to run custom build command for `proc-macro-error-attr v0.4.10`

Caused by:
  process didn't exit successfully: `/tmp/cargo-installkguS8W/release/build/proc-macro-error-attr-61231f39c0e665d6/build-script-build` (exit code: 101)
--- stderr
thread 'main' panicked at 'called `Option::unwrap()` on a `None` value', src/libcore/option.rs:378:21
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace.

warning: build failed, waiting for other jobs to finish...
error: failed to compile `cargo-udeps v0.1.7`, intermediate artifacts can be found at `/tmp/cargo-installkguS8W`

Caused by:
  build failed

This is probably because version_check::triple() returns None in the build.rs when the version does not contain a date.