Calculator: uses the correct classes

Fixes https://github.com/duckduckgo/zeroclickinfo-goodies/issues/567

![image](https://cloud.githubusercontent.com/assets/978048/3754297/a18563fe-1819-11e4-8e2f-8089aa2d4676.png)

![image](https://cloud.githubusercontent.com/assets/978048/3754300/ad62a40c-1819-11e4-938e-970cbb473f77.png)