travis: -Z minimal-versions flag for cargo build

.travis.yml

@@ -43,8 +43,8 @@ script:
   # we set release as build type for all steps to benefit from already compiled packages in prior steps
   # skipping clippy ...
   # - cargo +nightly clippy --release --target=${TARGET} -- -D warnings
-  - cargo +nightly test --release --verbose --all --target=${TARGET}
-  - WASM_BUILD_TOOLCHAIN=nightly cargo +nightly build --release --target=${TARGET}
+  - cargo +nightly -Z minimal-versions test --release --verbose --all --target=${TARGET}
+  - WASM_BUILD_TOOLCHAIN=nightly cargo +nightly -Z minimal-versions build --release --target=${TARGET}
 
 before_deploy: