fix for Boost >= 1.56

fix for Boost >= 1.56

Summary: Folly still continue to build and work on earlier boost versions(boost < 1.56).

**boost::context::make_fcontext() change history:**
* __1.55:__    fcontext_t * make_fcontext( void * sp, std::size_t size, void(* fn)(intptr_t) );
* __1.56:__    fcontext_t make_fcontext(void* sp,std::size_t size,void(*fn)(intptr_t));

**boost doc:**
http://www.boost.org/doc/libs/1_55_0/libs/context/doc/html/context/context/boost_fcontext.html
http://www.boost.org/doc/libs/1_56_0/libs/context/doc/html/context/context/boost_fcontext.html
Closes #205
Closes #210

Reviewed By: @jwatzman

Differential Revision: D2148069

Pulled By: @sgolemon