`spawn` error when generating output file

Hi,

Thanks for your work on this gem.

I am running into an error on Windows 10 using your gem. Below is my pseudo code -

```
g = Graphviz::Graph.new

# Add nodes

# Add connections

Graphviz::output(g, path: 'file')
```

Here's the error output -

```
Traceback (most recent call last):
5: from C:/tools/ruby25/lib/ruby/gems/2.5.0/gems/process-group-1.1.0/lib/process/group.rb:127:in `block in run'
4: from C:/tools/ruby25/lib/ruby/gems/2.5.0/gems/process-group-1.1.0/lib/process/group.rb:135:in `spawn'
3: from C:/tools/ruby25/lib/ruby/gems/2.5.0/gems/process-group-1.1.0/lib/process/group.rb:230:in `append!'
2: from C:/tools/ruby25/lib/ruby/gems/2.5.0/gems/process-group-1.1.0/lib/process/group.rb:241:in `schedule!'
1: from C:/tools/ruby25/lib/ruby/gems/2.5.0/gems/process-group-1.1.0/lib/process/group.rb:45:in `run'
C:/tools/ruby25/lib/ruby/gems/2.5.0/gems/process-group-1.1.0/lib/process/group.rb:45:in `spawn': wrong exec option symbol: pgroup (ArgumentError)
```

As a workaround, I am writing the dot contents manually like so -

`File.write(g.to_dot)`

Reproduce Graphviz::output on Windows 10 with the shown pseudo-code and Ruby 2.5, then inspect the process-group.rb stack frames around spawn and append!. Done means generating the output file no longer raises the pgroup ArgumentError, without requiring the File.write(g.to_dot) workaround.

Written by the indexing model from the issue text.