fixes #743. Instead of PI, multiplied by 2 and called it TWO_PI
Merge branch 'develop' into feature/issue-743-von_mises-pi
replaced TWO_PI with 2*stan::math::pi()
Update von_mises.hpp
Merge pull request #744 from stan-dev/feature/issue-743-von_mises-pi

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