fixed test_bcast.jl

In Julia0.4-dev, we can build MPI.jl but fail to test.

    The following tests FAILED:
          1 - test_bcast.jl_np:2 (Failed)
          4 - test_bcast.jl_np:5 (Failed)
    Errors while running CTest

This is caused by the change of behavior of integer conversion functions; Now these throws `InexactError()` when an overflow occurs,  for example, `int8(128)`.

see https://github.com/JuliaLang/julia/pull/8420.