PHPLIB-530: Test against MongoDB 4.3

Author: alcaeus

.travis.yml

@@ -91,6 +91,12 @@ jobs:
       env:
         - SERVER_VERSION=4.0.12
 
+    # Test upcoming server version
+    - stage: Test
+      php: "7.3"
+      env:
+        - SERVER_VERSION=4.3.3
+
     # Test other server configurations
     - stage: Test
       php: "7.3"