Remove cwd option

Fixes:

Running "wiredep:app" (wiredep) task
Warning: ENOENT, no such file or directory '/home/rhalff/git/Closery/loopback-angular-admin/client/app/bower.json'

See: http://stackoverflow.com/questions/25709324/grunt-wiredepapp-no-such-file-or-directory-bower-json