Fix spawn EINVAL error on Windows

[derevnjuk]

fixes #83

[vweevers]

LGTM but @mafintosh's comment here makes me unsure: #81 (comment)

[derevnjuk]

@vweevers If so, it is better to enforce shell in this particular case like this:

-var child = execspawn(cmd, {
+var child = proc.spawn(cmd, [], {
  cwd: opts.cwd,
  env: opts.env,
  stdio: 'inherit',
-  shell: opts.shell
+  shell: opts.shell || true,
+  windowsHide: true
})

c14ed8b wdyt?

[vweevers]

6.0.1