fixes #743. Instead of PI, multiplied by 2 and called it TWO_PI

#### Summary:

`PI` is used as a macro somewhere in RStan's preprocessed chain (could be from anywhere). This conflicts with our definition of von_mises.

This fix just replaces PI with TWO_PI (by multiplying by two).

#### Intended Effect:

Allows RStan to build without affecting Stan.

#### How to Verify:

Tests should all be the same.

#### Side Effects:

None.

#### Documentation:

Internal change.

#### Reviewer Suggestions:

Anyone.