Remove mongodb repo file from build host

See travis-ci/travis-ci#8581 (comment)

Author: Robert Kratky

.travis.yml

@@ -8,6 +8,7 @@ services:
   - docker
 
 before_install:
+  - sudo rm /etc/apt/sources.list.d/mongodb-3.2.list
   - mkdir -p output
   - docker pull asciidoctor/docker-asciidoctor