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
```

There is a `--enable-java` configure option. Are you picturing something like:
```ruby
depends_on :java => :optional
…
args << '--disable-java' if build.without? 'java'
…
if build.with? 'java'
  libexec.install bin/'vtsetup.jar'
  inreplace bin/'vtsetup', '$bindir/vtsetup.jar', '$prefix/libexec/vtsetup.jar'
end
```