Fix angle mode setting in calculator

Summary:
Due to the way JS files are packaged, it seems that `Calculator` inside the closure was referring to the old calculator object instead of the wrapper we make around it. Just avoid referencing it to avoid any confusion.

Should fix trig functions not working on the live site.

Reviewers: eater

Reviewed By: eater

Differential Revision: http://phabricator.khanacademy.org/D5759