Switch from manual steps to apt setup script

Encapsulating this stuff will make it easier for other extensions
to follow suit on 9.5.

.travis.yml

@@ -10,11 +10,8 @@ env:
     - PGVERSION=9.5
     - PGVERSION=40.0
 before_install:
-  - wget --quiet -O - https://www.postgresql.org/media/keys/ACCC4CF8.asc | sudo apt-key add -
-  - sudo sh -c 'echo "deb http://apt.postgresql.org/pub/repos/apt/ precise-pgdg main 9.5" >> /etc/apt/sources.list.d/postgresql.list'
-  - sudo apt-get update -qq
-  - sudo update-alternatives --remove-all postmaster.1.gz
   - git clone --depth 1 https://gist.github.com/9963879.git tools
+  - tools/setup_apt.sh
   - tools/nuke_pg.sh
 install:
   - sudo pip install cpp-coveralls