add mongodb 4.0 to travis and docs

.travis.yml

@@ -3,7 +3,7 @@
 # with a very large number of jobs, hence we only test a subset of all the
 # combinations:
 # * MongoDB v3.4 & the latest PyMongo v3.x is currently the "main" setup,
-#   tested against Python v2.7, v3.5, v3.6, and PyPy.
+#   tested against Python v2.7, v3.5, v3.6, v3.7, PyPy and PyPy3.
 # * Besides that, we test the lowest actively supported Python/MongoDB/PyMongo
 #   combination: MongoDB v3.4, PyMongo v3.4, Python v2.7.
 # * MongoDB v3.6 is tested against Python v3.6, and PyMongo v3.6, v3.7, v3.8.
@@ -30,15 +30,16 @@ dist: xenial
 
 env:
   global:
+    - MONGODB_4_0=4.0.12
     - MONGODB_3_4=3.4.17
     - MONGODB_3_6=3.6.12
+    - PYMONGO_3_9=3.9
     - PYMONGO_3_6=3.6
     - PYMONGO_3_4=3.4
   matrix:
-    - MONGODB=${MONGODB_3_4} PYMONGO=${PYMONGO_3_6}
+    - MONGODB=${MONGODB_3_4} PYMONGO=${PYMONGO_3_9}
 
 matrix:
-
   # Finish the build as soon as one job fails
   fast_finish: true
 
@@ -47,7 +48,8 @@ matrix:
     env: MONGODB=${MONGODB_3_4} PYMONGO=${PYMONGO_3_4}
   - python: 3.7
     env: MONGODB=${MONGODB_3_6} PYMONGO=${PYMONGO_3_6}
-
+  - python: 3.7
+    env: MONGODB=${MONGODB_4_0} PYMONGO=${PYMONGO_3_9}
 
 install:
   # Install Mongo