Install Varnish from source in Travis CI

The Varnish version appears only once, making it hard to break on
Varnish upgrades.

.travis.yml

+---
+sudo: required
+dist: trusty
+
 language: c
 
-before_install:
-  - sudo apt-get update -q
-  - sudo apt-get install -qq apt-transport-https python-docutils
-  - curl https://repo.varnish-cache.org/debian/GPG-key.txt | sudo apt-key add -
-  - echo "deb https://repo.varnish-cache.org/ubuntu/ precise varnish-4.1" | sudo tee /etc/apt/sources.list.d/varnish-cache.list
-  - sudo apt-get -q update
-  - sudo apt-get install varnish libvarnishapi-dev
+addons:
+  apt:
+    packages:
+    - python-docutils
 
+before_install:
+  - set -e
+  - wget https://repo.varnish-cache.org/source/varnish-5.0.0.tar.gz
+  - tar -zxf varnish-*.tar.gz
+  - pushd varnish-*/
+  - ./configure --prefix=/usr
+  - make -sj32
+  - sudo make install
+  - popd
 
 before_script:
   - ./autogen.sh