add ipython to dependencies for RTD
add ipython to dependencies for RTD

RTD build is currently failing with 
```
ExtensionError: Could not import extension IPython.sphinxext.ipython_console_highlighting (exception: No module named IPython.sphinxext.ipython_console_highlighting)
```
cjcardinale/climlab,brian-rose/climlab,cjcardinale/climlab,cjcardinale/climlab,brian-rose/climlab