message: Finish pushing the minimum Rust to 1.31.0
diff: @@ -25,5 +25,5 @@ test_script:
  # we don't have ci time to run the full `cargo test` with `minimal-versions` like
  # - if defined MINIMAL_VERSIONS cargo +nightly generate-lockfile -Z minimal-versions && cargo +stable test
  # so we just run `cargo check --tests` like
  - - if defined MINIMAL_VERSIONS cargo +nightly generate-lockfile -Z minimal-versions && cargo +1.28.0 check --tests
  + - if defined MINIMAL_VERSIONS cargo +nightly generate-lockfile -Z minimal-versions && cargo +1.31.0 check --tests
  - if NOT defined MINIMAL_VERSIONS cargo test
  