Appveyor: install graphviz

I don't know whether I should be using

  choco install graphviz

or

  choco install graphviz.portable

and https://github.com/chocolatey/choco/wiki/ChocolateyFAQs#what-distinction-does-chocolatey-make-between-an-installable-and-a-portable-application
didn't help me decide, so I'll try the shorter name first.

appveyor.yml

@@ -17,6 +17,8 @@ install:
   - "set PATH=%PYTHON%;%PYTHON%\\Scripts;%PATH%"
   - python --version
   - pip install tox
+  - choco install graphviz
+  - dot -V
 
 build: off