Test server 4.0 by default in Travis and bump older versions

.travis.yml

@@ -16,7 +16,7 @@ cache:
 env:
   global:
     - DRIVER_VERSION=1.5.0
-    - SERVER_VERSION=3.6.2
+    - SERVER_VERSION=4.0.0
 
 matrix:
   fast_finish: true
@@ -31,10 +31,13 @@ matrix:
         - SERVER_VERSION=3.0.15
     - php: 7.0
       env:
-        - SERVER_VERSION=3.2.18
+        - SERVER_VERSION=3.2.20
     - php: 7.0
       env:
-        - SERVER_VERSION=3.4.11
+        - SERVER_VERSION=3.4.15
+    - php: 7.0
+      env:
+        - SERVER_VERSION=3.6.5
 
 before_install:
   - pip install "mongo-orchestration>=0.6.7,<1.0" --user `whoami`