updated travis config file, now the correct mongodb are used.

Author: zookzook

.travis.yml

@@ -32,7 +32,7 @@ script:
 
 env:
   matrix:
-#    - MONGODB=3.2.20
-#    - MONGODB=3.4.16
+    - MONGODB=3.2.20
+    - MONGODB=3.4.16
     - MONGODB=3.6.6
-#    - MONGODB=4.0.9
+    - MONGODB=4.0.9