message: Add build on v1.23 of Rust.
diff: @@ -36,6 +36,13 @@ matrix:
  apt:
  packages:
  - gnuplot
  + - os: linux
  + env: GNUPLOT=yes
  + rust: 1.23.0
  + addons:
  + apt:
  + packages:
  + - gnuplot
  - os: linux
  env: CLIPPY=yes
  rust: nightly-2018-01-12
  