bump(luajit): Bump Luajit to 2.0.3

- Notible fixes (http://luajit.org/changes.html)

  * Fix frame traversal for backtraces
  * FFI: Add missing GC steps for callback argument conversions
Merge pull request #449 from luvit/bump/luajit_2_0_3

bump(luajit): Bump Luajit to 2.0.3