Added case to typeof_global for None global value

Sometimes (for large functions), `None` uses the `LOAD_GLOBAL` instruction instead of the `LOAD_CONST` instruction, as Numba currently assumes.  This change will catch the case that `None` is loaded as a global var.
Fixes regression with ExternalFunctionDescriptor
Added impyla to travis
Merge branch 'impyla-ci' of https://github.com/laserson/numba (#454)
Merge branch 'ext-fn-desc' of https://github.com/laserson/numba (#460)
Merge branch 'global_none' of https://github.com/laserson/numba (#425)