chore: switch to latest 4x version on fastdl downloads

Author: saintedlama

.travis.yml

@@ -16,7 +16,7 @@ env:
   - MONGODB_VERSION="3.2.16"
   - MONGODB_VERSION="3.4.7"
   - MONGODB_VERSION="3.6.2"
-  - MONGODB_VERSION="4.2.2"
+  - MONGODB_VERSION="4.1.6"
 
 before_script:
   - wget http://fastdl.mongodb.org/linux/mongodb-linux-x86_64-${MONGODB_VERSION}.tgz -O /tmp/mongodb.tgz