Restore no-default-features test on nightly

PR 192 changed `cargo test --no-default-features` to `cargo test
--no-default-features -- --ignored`. But `--ignored` runs *only* the
ignored tests. We want to run the non-ignored tests also.
diff --git a/.travis.yml b/.travis.yml
index 081123b..acddb57 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -9,6 +9,7 @@
     - rust: nightly
       script:
         - cargo test
+        - cargo test --no-default-features
         - cargo test --no-default-features -- --ignored # run the ignored test to make sure the `proc-macro` feature is disabled
         - cargo test --features span-locations
         - RUSTFLAGS='--cfg procmacro2_semver_exempt' cargo test