It turns out that Busted started cleaning the environment in 2.0rc5 as a
result of Olivine-Labs/busted#62. This, in turn, caused the ffi module
to be reloaded for each spec file, and LuaJIT doesn't appreciate it.
The net effect is an assertion error in LuaJIT.
By using the --helper feature of Busted, we can pre-load some modules
ahead of Busted and prevent it from reloading them--making LuaJIT happy
again.
