Try to install graphviz so we can actually test d3viz

.travis/travis_install.sh

@@ -9,6 +9,6 @@ else
 fi
 
 source activate pyenv
-if [[ $TRAVIS_PYTHON_VERSION == '2.7' ]]; then conda install --yes -q mkl numpy=1.9.1 scipy=0.14.0 nose=1.3.0 pip flake8=2.3 six=1.9.0 pep8=1.6.2 pyflakes=0.8.1 sphinx=1.5.1 mkl-service libgfortran=1; fi
-if [[ $TRAVIS_PYTHON_VERSION == '3.4' ]]; then conda install --yes -q mkl numpy=1.9.1 scipy=0.14.0 nose=1.3.4 pip flake8=2.3 six=1.9.0 pep8=1.6.2 pyflakes=0.8.1 sphinx=1.5.1 mkl-service libgfortran=1; fi
+if [[ $TRAVIS_PYTHON_VERSION == '2.7' ]]; then conda install --yes -q mkl numpy=1.9.1 scipy=0.14.0 nose=1.3.0 pip flake8=2.3 six=1.9.0 pep8=1.6.2 pyflakes=0.8.1 sphinx=1.5.1 mkl-service libgfortran=1 graphviz; fi
+if [[ $TRAVIS_PYTHON_VERSION == '3.4' ]]; then conda install --yes -q mkl numpy=1.9.1 scipy=0.14.0 nose=1.3.4 pip flake8=2.3 six=1.9.0 pep8=1.6.2 pyflakes=0.8.1 sphinx=1.5.1 mkl-service libgfortran=1 graphviz; fi
 source deactivate