Grunt error

grunt.js

module.exports = function(grunt) {
  grunt.loadNpmTasks('grunt-exec');

  grunt.initConfig({
    exec: {
      build: {
        command: 'node node_modules/requirejs/bin/r.js -o require-config.js'
      }
    }
  });

  grunt.registerTask('copy-require', function() {
    grunt.file.mkdir('build/js/lib');
    grunt.file.copy('node_modules/requirejs/require.js', 'build/js/lib/require.js');
  });

  grunt.registerTask('default', 'exec copy-require');
};

package.json

{
  "name": "derp"
, "version": "0.0.1"
, "private": true
, "dependencies": {
    "requirejs": "latest"
  , "connect": "2.7.0"
  }
, "devDependencies": {
    "mocha": "latest"
  , "chai": "latest"
  , "grunt": "latest"
  , "grunt-exec": "latest"
  }
, "scripts": {
    "grunt": "node_modules/.bin/grunt"
  }
}

require-config.js

({
  appDir: 'app/'
, baseUrl: '/js'
, paths: {}
, dir: 'build/'
, modules: [{ name: 'main' }]
})

running with just node_modules-bin-grunt

$ node_modules/.bin/grunt
Running "exec:build" (exec) task
<WARN> Command failed:  Use --force to continue. </WARN>

Aborted due to warnings.

terminal output

$ npm run-script grunt

> derp@0.0.1 grunt /Users/pkarl/Sites/backboners
> node_modules/.bin/grunt

Running "exec:build" (exec) task
<WARN> Command failed:  Use --force to continue. </WARN>

Aborted due to warnings.
npm ERR! derp@0.0.1 grunt: `node_modules/.bin/grunt`
npm ERR! `sh "-c" "node_modules/.bin/grunt"` failed with 2
npm ERR! 
npm ERR! Failed at the derp@0.0.1 grunt script.
npm ERR! This is most likely a problem with the derp package,
npm ERR! not with npm itself.
npm ERR! Tell the author that this fails on your system:
npm ERR!     node_modules/.bin/grunt
npm ERR! You can get their info via:
npm ERR!     npm owner ls derp
npm ERR! There is likely additional logging output above.

npm ERR! System Darwin 12.2.1
npm ERR! command "node" "/usr/local/bin/npm" "run-script" "grunt"
npm ERR! cwd /Users/pkarl/Sites/backboners
npm ERR! node -v v0.8.15
npm ERR! npm -v 1.1.66
npm ERR! code ELIFECYCLE
npm ERR! 
npm ERR! Additional logging details can be found in:
npm ERR!     /Users/pkarl/Sites/backboners/npm-debug.log
npm ERR! not ok code 0