Run exhaustive test suite on 1.43+ only

The `self: &T` syntax in extern functions doesn't work yet in 1.42. The
intro demo code should work though.
diff --git a/.travis.yml b/.travis.yml
index 28ad110..49bacbd 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -3,7 +3,6 @@
 rust:
   - nightly
   - beta
-  - stable
 
 script:
   - cargo run --manifest-path demo-rs/Cargo.toml
@@ -55,3 +54,5 @@
         - bazel test ... --verbose_failures --noshow_progress
     - name: Minimum rustc
       rust: 1.42.0
+      script:
+        - cargo run --manifest-path demo-rs/Cargo.toml