Use raw troff encoding for official HHVM man page

Summary: So we don't have to require `ronn` to build the man page, just use the raw troff
format instead. That way it can be put into `/usr/share/man/man1` directly and just
work.

Reviewed By: @ptarjan

Differential Revision: D1536269
facebook::proxygen --> ::proxygen

Summary: See discussion here: https://www.facebook.com/groups/opensourceatfb/permalink/771563229554092/
After I get fbcode contbuild clean, I'll submit a diff for
hhvm-third-party. In the meantime, the cmake build will fail here.

Reviewed By: @ptarjan

Differential Revision: D1537217