only test minimal-versions on nightly

Author: Eh2406

.travis.yml

@@ -4,12 +4,12 @@ rust:
   - stable
   - beta
   - nightly
-before_script:
-  - rustup toolchain install nightly
 script:
   - cargo build --verbose
   - cargo test --verbose
   - cargo doc
-  - cargo +nightly generate-lockfile -Z minimal-versions
-  - cargo build --verbose
-  - cargo test --verbose
+  - if [ "$TRAVIS_RUST_VERSION" = "nightly" ]; then
+    cargo +nightly generate-lockfile -Z minimal-versions;
+    cargo build --verbose;
+    cargo test --verbose;
+    fi