Only test serde feature on stable to save CI resources.

.travis.yml

@@ -7,14 +7,18 @@ rust:
 
 env:
   - TEST_COMMAND=test FEATURES=''
-  - TEST_COMMAND=test FEATURES='--features=serde'
 
 matrix:
   include:
+    # We use the 64-bit optimised curve backend by default, so also test with the 32-bit backend:
     - rust: nightly
       env: TEST_COMMAND=build FEATURES='--no-default-features --features=u32_backend'
+    # Test any nightly gated features on nightly:
     - rust: nightly
       env: TEST_COMMAND=test FEATURES='--features=nightly'
+    # Test serde support on stable, assuming that if it works there it'll work everywhere:
+    - rust: stable
+      env: TEST_COMMAND=test FEATURE='--features=serde'
 
 script:
   - cargo $TEST_COMMAND $FEATURES