Fix travis documentation tests

The documentation fails to build but the test passes. The scripting
is to blame, it does not bailout after the failing line.

.travis.yml

@@ -1,4 +1,4 @@
-sudo: false
+sudo: required
 
 language: python
 
@@ -40,6 +40,11 @@ before_install:
     else
       wget https://repo.continuum.io/miniconda/Miniconda3-latest-Linux-x86_64.sh -O miniconda.sh
     fi
+  - |
+    if [[ "$JOB" == "DOCS" ]]; then
+      sudo apt-get -qq update
+      sudo apt-get install -y pandoc
+    fi
   - bash miniconda.sh -b -p $HOME/miniconda
   - export PATH="$HOME/miniconda/bin:$PATH"
   - hash -r
@@ -81,8 +86,7 @@ script:
       make test
     elif [[ "$JOB" == "DOCS" ]]; then
       pushd doc
-      make html SPHINXOPTS=-W
-      popd;
+      make html SPHINXOPTS=-W && popd
     elif [[ "$JOB" == "LINT" ]]; then
       make lint
     fi

requirements_doc.txt

@@ -2,3 +2,4 @@ git+https://github.com/has2k1/plotnine-examples#egg=plotnine_examples
 jupyter
 sphinx>=1.6.1
 nbsphinx
+pillow