fix (previously unreachable?) no module found return value generation.

Correctness rationale:
1. "all logical operators consider false and nil as false and anything else as true" (http://www.lua.org/pil/3.3.html)
2. Lua complains "attempt to concatenate" for both boolean and nil values
∴. If the original code were reached, it would have exploded.

With this patch (and an upcoming patch) this gets us closer to an accurate error message when the module is not found, although there's still some [old?] folder-fallback logic in colony.run that gets us.
failing test for #276
change require() directory traversal logic to check topmost folder, fixes #276
Merge pull request #279 from natevw/issue-276

Fix require() parent directory traversal