- Fixed tests to allow support of MongoDB 3.2

- Replaced MongoDB 2.4 tests in CI by MongoDB 3.2

.travis.yml

@@ -27,17 +27,17 @@ matrix:
 
   include:
   - python: 2.7
-    env: MONGODB=2.4 PYMONGO=3.5
+    env: MONGODB=3.0 PYMONGO=3.5
   - python: 2.7
-    env: MONGODB=3.0 PYMONGO=3.x
+    env: MONGODB=3.2 PYMONGO=3.x
   - python: 3.5
-    env: MONGODB=2.4 PYMONGO=3.5
+    env: MONGODB=3.0 PYMONGO=3.5
   - python: 3.5
-    env: MONGODB=3.0 PYMONGO=3.x
+    env: MONGODB=3.2 PYMONGO=3.x
   - python: 3.6
-    env: MONGODB=2.4 PYMONGO=3.5
+    env: MONGODB=3.0 PYMONGO=3.5
   - python: 3.6
-    env: MONGODB=3.0 PYMONGO=3.x
+    env: MONGODB=3.2 PYMONGO=3.x
 
 before_install:
 - bash .install_mongodb_on_travis.sh