Test with -Z minimal-versions on CI

Author: hcpl

.travis.yml

@@ -10,6 +10,9 @@ before_script:
 script:
   - cargo run -p ci
   - cargo run --example default
+  - if [ "${TRAVIS_RUST_VERSION}" = nightly ]; then cargo update -Z minimal-versions; fi
+  - if [ "${TRAVIS_RUST_VERSION}" = nightly ]; then cargo run -p ci; fi
+  - if [ "${TRAVIS_RUST_VERSION}" = nightly ]; then cargo run --example default; fi
 after_success:
   - travis-cargo --only nightly doc-upload