Update checkForGitbook.R

npm test via system() returns 1, not only 0. Although, npm is installed and works fine on Mac OS X 10.8.5.
This fixes that problem, although I'm not sure it is the best solution. Temporarily, works for me.

![screenshot 2014-05-28 16 28 15](https://cloud.githubusercontent.com/assets/638225/3105578/55635192-e674-11e3-8482-f7f69d6fbe5d.png)