roll_cagr not working 

I am trying to do:
```
from empyrical import  roll_beta, roll_alpha, roll_sharpe_ratio, roll_annual_volatility, roll_cagr
```
but get an error message whereas the function seems to exists. I don't know why (the others functions are ok)
```
ImportError: cannot import name 'roll_cagr' from 'empyrical' 
```
thx for your help. 