Fix JRuby exception in posix run! method

```
NoMethodError:
undefined method `success?' for #<Process::WaitThread:0xd5c0e84 dead>
```

WaitThread does not have a success? method, but it does have a value
property which can be checked instead.

Build succeeds with [jruby-d19](https://travis-ci.org/justenwalker/librarian/jobs/16218343)