travis: install clang-format-3.5 from llvm repository

Ubuntu utopic disappeared from archive.ubuntu.com, it’s EOL.

.travis.yml

@@ -2,12 +2,20 @@ language: c
 compiler:
   - gcc
   - clang
+addons:
+  apt:
+    sources:
+    - llvm-toolchain-precise-3.5
+    # ubuntu-toolchain-r-test contains libstdc++6 >= 4.8 which libllvm3.5 needs.
+    - ubuntu-toolchain-r-test
+    packages:
+    - clang-format-3.5
+    - libllvm3.5
 before_install:
   # The travis VMs run on Ubuntu 12.04 which is very old and a huge pain to get
   # into a state where we can build a recent version of i3 :(.
   - "echo 'deb http://archive.ubuntu.com/ubuntu/ trusty main universe' | sudo tee /etc/apt/sources.list.d/trusty.list"
   - "echo 'APT::Default-Release \"precise\";' | sudo tee /etc/apt/apt.conf.d/default-release"
-  - "echo 'deb http://archive.ubuntu.com/ubuntu/ utopic main universe' | sudo tee /etc/apt/sources.list.d/utopic.list"
 
   - "echo 'Package: libc6' > /tmp/pin"
   - "echo 'Pin: release n=trusty' >> /tmp/pin"
@@ -33,8 +41,6 @@ before_install:
   - sudo apt-get update
   - sudo apt-get install -t trusty libc6 libc6-dev
   - sudo apt-get install --no-install-recommends devscripts equivs xdotool
-  - sudo apt-get install -t utopic clang-format-3.5
-  - clang-format-3.5 --version
 install:
   - sudo mk-build-deps --install --remove --tool 'apt-get --no-install-recommends' debian/control
   # Install as many dependencies as possible via apt because cpanm is not very reliable/easy to debug.