Add dvipng package

.travis.yml

@@ -28,6 +28,7 @@ addons:
    - texlive-latex-recommended
    - texlive-latex-extra
    - texlive-fonts-recommended
+   - dvipng
 
 install:
   - if [[ $TRAVIS_PYTHON_VERSION == '2.6' ]]; then conda create --yes -q -n pyenv mkl python=2.6 numpy=1.7.1 scipy=0.11 nose=1.3.0 pyparsing=1.5 pip flake8=2.3 six=1.9.0 pep8=1.6.2 pyflakes=0.8.1; fi