fixed test_bcast.jl

[yomichi]

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 JuliaLang/julia#8420.

[lcw]

Thanks!