Update rethinkdb.rb

https://rethinkdb.com/blog/download-server-changes https://rethinkdb.com/blog/download-server-changes.

Author: mustafa-travisci

lib/travis/build/addons/rethinkdb.rb

@@ -16,7 +16,7 @@ def after_prepare
               sh.echo "Installing RethinkDB version #{rethinkdb_version}", ansi: :yellow
               sh.cmd "service rethinkdb stop", sudo: true
               sh.cmd "sudo apt-key adv --keyserver pgp.mit.edu --recv-keys \"539A 3A8C 6692 E6E3 F69B 3FE8 1D85 E93F 801B B43F\"", echo: true
-              sh.cmd 'echo -e "\ndeb https://download.rethinkdb.com/repository/ubuntu-$(lsb_release -cs)/ $(lsb_release -cs) main" | sudo tee -a /etc/apt/sources.list > /dev/null'
+              sh.cmd 'echo -e "\ndeb https://download.rethinkdb.com/repository/ubuntu-$(lsb_release -cs)/ $(lsb_release -cs) main" | sudo tee -a /etc/apt/sources.list.d/rethinkdb.list > /dev/null'
               sh.cmd 'travis_apt_get_update', assert: false
               sh.cmd "package_version=`apt-cache show rethinkdb | grep -F \"Version: #{rethinkdb_version}\" | sort -r | head -n 1 | awk '{printf $2}'`"
               sh.cmd "apt-get install -y -o Dpkg::Options::='--force-confnew' rethinkdb=$package_version", sudo: true, echo: true, timing: true