Add graphviz to Binder image using apt.txt API (#3669)

* Add graphviz to Binder image using apt.txt API

graphviz is needed for pymc3.model_to_graphviz and is used in some
example notebooks. If Binder finds an apt.txt file it will then proceed
to install its contents with apt-get in the image it builds with
repo2docker.

c.f. https://github.com/binder-examples/latex

* Update Binder badge to use new badge design

Author: matthewfeickert

README.rst

@@ -186,7 +186,7 @@ Sponsors
 
 |ODSC|
 
-.. |Binder| image:: https://mybinder.org/badge.svg
+.. |Binder| image:: https://mybinder.org/badge_logo.svg
    :target: https://mybinder.org/v2/gh/pymc-devs/pymc3/master?filepath=%2Fdocs%2Fsource%2Fnotebooks
 .. |Build Status| image:: https://travis-ci.org/pymc-devs/pymc3.svg?branch=master
    :target: https://travis-ci.org/pymc-devs/pymc3

binder/apt.txt

@@ -0,0 +1 @@
+graphviz