test(travis): Test build with minimal versions

Author: klausi

.travis.yml

@@ -35,6 +35,9 @@ matrix:
         - cargo test
         - cargo doc --no-deps
         - (cd src/doc && mdbook build --dest-dir ../../target/doc)
+        # Make sure building cargo works with minimum dependency versions.
+        - ./target/debug/cargo update -Z minimal-versions
+        - cargo build
 
   exclude:
     - rust: stable