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
```

`depends_on :java => :optional` doesn't actually work because `:java` is a symbol rather than a string, but I can imitate it. Could you add support for symbols to `depends_on … => :optional`?