Summary: As per http://en.cppreference.com/w/cpp/numeric/random/uniform_int_distribution the behavior of using `uint8_t` as the template parameter is undefined, and is not supported on MSVC, so use `unsigned short` instead, which is a defined form.

Reviewed By: yfeldblum

Differential Revision: D3507309

fbshipit-source-id: c4c830371d08aee4a3de90bb394d22d92ad9a575