Make npm executable

Client manifests can't use npm to install node.js dependencies unless the npm script is executable.

Ex. Will raise `Can't find program /usr/local/node/node-default/bin/npm`

```ruby
exec { 'npm-install':
    command => '/usr/local/node/node-default/bin/npm install -d',
    cwd         => $application_directory,
  }
```