Fix build with certain MPI implementations
Fix build with certain MPI implementations

Removing the `IMPLICIT NONE` statement which doesn't play nice will all
MPI implementations.  Thanks Jeff Bezanson for the report, this fixes #1.

JuliaParallel/MPI.jl,JuliaPackageMirrors/MPI.jl,pressel/MPI.jl,staticfloat/MPI.jl,jkozdon/MPI.jl