TravisCI: Specify custom MongoDB version

.travis.yml

# e.g. for MongoDB 2.6
#
# Follows MongoDB for Ubuntu installation https://docs.mongodb.com/v2.6/tutorial/install-mongodb-on-ubuntu/
#   note: `--allow-downgrades` flag for apt-get install may be required

language: scala

scala: 2.11.11

jdk: openjdk8

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=2.6.12 mongodb-org-server=2.6.12 mongodb-org-shell=2.6.12 mongodb-org-mongos=2.6.12 mongodb-org-tools=2.6.12 --allow-downgrades
- sleep 15 # https://docs.travis-ci.com/user/database-setup/#MongoDB-does-not-immediately-accept-connections

script:
- sbt ++$TRAVIS_SCALA_VERSION test

after_script:
- pkill mongod