Add graphviz dep to Travis

Author: ashgillman

.travis.yml

@@ -33,7 +33,7 @@ install:
   conda config --add channels conda-forge &&
   conda update --yes conda &&
   conda update --all -y python=$TRAVIS_PYTHON_VERSION &&
-  conda install -y nipype &&
+  conda install -y nipype graphviz &&
   rm -r /home/travis/miniconda/lib/python${TRAVIS_PYTHON_VERSION}/site-packages/nipype* &&
   pip install -r requirements.txt &&
   pip install -e .[$NIPYPE_EXTRAS] &&