Fix build by removing reference to obsolete and no longer unneeded PPA (only 12.04 needed it)

Author: fidergo-stephane-gourichon

.travis.yml

@@ -3,8 +3,6 @@ python:
   - 2.6
   - 2.7
 install:
-  # 12.04 LTS ships an outdated texlive https://launchpad.net/bugs/712521
-  - sudo add-apt-repository --yes ppa:texlive-backports/ppa
   - sudo apt-get update -yq2
   - sudo apt-get install -yq2 --no-install-recommends dvipng graphviz imagemagick lilypond source-highlight texlive-latex-base
 script: