Update to fix issue in Travis

```
ruby/2.0.0/gems/backports-3.6.0/lib/backports/1.9.1/io/open.rb:2:in `close': Bad file descriptor - /app/vendor/bundle/ruby/2.0.0/gems/backports-3.6.0/lib/backports/1.9.1/io/open.rb (Errno::EBADF)
```

See https://github.com/marcandre/backports/issues/86 for details.