.travis.yml: Add pinned vivid for gettext and clean up a bit

This adds a vivid pinned to -1, cleans up the file a bit by
removing duplicate commands, and finally installs gettext
with a new apt-get run that is passed -t vivid.

The syntax for the pinning is some weird YAML stuff I don't
want to think about...

.travis.yml

@@ -2,12 +2,14 @@ language: cpp
 sudo: required
 before_install:
  - sudo add-apt-repository ppa:ubuntu-toolchain-r/test -y
+ - sudo add-apt-repository 'deb http://archive.ubuntu.com/ubuntu/ vivid main' -y
+ - |
+   sudo sh -c '/bin/echo -e "Package: *\nPin: release n=vivid\nPin-Priority: 1" > /etc/apt/preferences.d/vivid'
  - sudo apt-get update -qq
 install:
  - sudo ./prepare-release travis-ci
- - sudo add-apt-repository -y ppa:ubuntu-toolchain-r/test
- - sudo apt-get -qq update
  - sudo apt-get -qq -y install gcc-4.8 g++-4.8
+ - sudo apt-get -qq -y -t vivid install gettext
  - export CC=gcc-4.8 CXX=g++-4.8
 script:
  - make