RuntimeError: Failed to import pydot

summarize:

1. install C-version of graphviz using 'sudo apt-get install graphviz' if ubuntu, 'brew install graphviz' if OSX
2. pip install pydot ( if it fails to import pydot in later 
3. pip install graphviz

cite: https://github.com/Theano/Theano/issues/1801