Add texlive-fonts-extra package to travis

This package is required for mathabx, which is included in the
header, but not acutally used in this document :-(

.travis.yml

@@ -1,6 +1,6 @@
 before_install:
 - sudo apt-get update
-- sudo apt-get install -y texlive-latex-extra texlive-lang-german texlive-math-extra
+- sudo apt-get install -y texlive-latex-extra texlive-lang-german texlive-math-extra texlive-fonts-extra
 script:
 - pdflatex -interaction=nonstopmode ./PmSwEng.tex
 - pdflatex -interaction=nonstopmode ./PmSwEng.tex