TravisCI: change dist to xenial

- Change distribution to `xenial`
- Remove custom mongodb 4.0 installation since xenial ships with 4.0
- Increase wait time from 5 to 15 seconds

.travis.yml

@@ -1,8 +1,8 @@
 language: clojure
 sudo: required
 lein: lein
+dist: xenial
 before_script:
-  - ./bin/ci/install_mongodb.sh
   - mongod --version
   - ./bin/ci/before_script.sh
 script: lein do clean, javac, test

bin/ci/before_script.sh

@@ -1,7 +1,7 @@
 #!/bin/sh
 
 # MongoDB seems to need some time to boot first. MK.
-sleep 5
+sleep 15
 
 # MongoDB Java driver won't run authentication twice on the same DB instance,
 # so we need to use multiple DBs.