Run Travis tests for multiple MongoDB versions

Repeating same environment variable causes TravisCI to run tests with both MongoDB versions on each NodeJS environment.

I'm not sure if this is too much testing but hey one can never be too careful, especially since MongoDB tends to move kinda fast and deprecate features often.

Docs: https://docs.travis-ci.com/user/environment-variables/

.travis.yml

@@ -8,11 +8,15 @@ node_js:
   - "7"
   - "8"
 script: make test-once
+env:
+  matrix:
+    - MONGODB_VER=2.6.6
+    - MONGODB_VER=3.4.6
 before_script:
   - sudo apt-key adv --keyserver hkp://keyserver.ubuntu.com:80 --recv 7F0CEB10
   - echo 'deb http://downloads-distro.mongodb.org/repo/ubuntu-upstart dist 10gen' | sudo tee /etc/apt/sources.list.d/mongodb.list
   - sudo apt-get update
-  - sudo apt-get install -y mongodb-org=2.6.6 mongodb-org-server=2.6.6 mongodb-org-shell=2.6.6 mongodb-org-mongos=2.6.6 mongodb-org-tools=2.6.6
+  - sudo apt-get install -y mongodb-org=$MONGODB_VER mongodb-org-server=$MONGODB_VER mongodb-org-shell=$MONGODB_VER mongodb-org-mongos=$MONGODB_VER mongodb-org-tools=$MONGODB_VER
   - sleep 15 #mongo may not be responded directly. See http://docs.travis-ci.com/user/database-setup/#MongoDB
   - mongo --version
 after_script: make test-coveralls