Fixing a bug on the 'opts' attribute on the first argument to grunt.util.spawn.

The 'opts' property on ISpawnOptions specifies "Additional options for the Node.js child_process spawn method." according to http://gruntjs.com/api/grunt.util#grunt.util.spawn

However, the previous typing incorrectly recursively typed this as ISpawnOptions. child_process.spawn takes different arguments from grunt.util.spawn.

I have copied in the appropriate typing from node/node.d.ts, as it is not an exported interface type. A quick inspection of the grunt source verifies that this object is passed uninspected to child_process.spawn, so all are potentially relevant.
Merge pull request #1599 from jvilk/grunt-fixes

[gruntjs] Fixing grunt.util.spawn 'opts' argument attribute