Remove iplot warnings in python3

@theengineear @chriddyp 

Fixes https://github.com/plotly/python-api/issues/155


I ran into warnings without the `'replace'` arg (see [here](https://docs.python.org/3.0/howto/unicode.html#the-string-type)). 

![image](https://cloud.githubusercontent.com/assets/6675409/5110589/eef76044-6fe7-11e4-877f-02df0260c585.png)