open-mpi 1.8.3: Fix the bin/vtsetup script

The `bin/vtsetup` script does not work, because `$bindir/vtsetup.jar` is expected to be a jar file, not a shell script.

```
❯❯❯ vtsetup
Error: Invalid or corrupt jarfile /usr/local/Cellar/open-mpi/1.8.3/bin/vtsetup.jar
```

`/usr/bin/java` will always exist. Is there a configure option for that? Would rather make this a `build.with?` if possible.