Remove ShellJS copy and use the NPM version

There is no need to have a copy of ShellJS in the repository as it is also available on NPM. The NPM version is also much newer. This way we do not have to update this anymore and let NPM do that automatically.

make.js

@@ -20,7 +20,14 @@
 
 'use strict';
 
-require('./external/shelljs/make');
+try {
+  require('shelljs/make');
+} catch (e) {
+  console.log('ShellJS is not installed. Run "npm install" to install ' +
+              'all dependencies.');
+  return;
+}
+
 var builder = require('./external/builder/builder.js');
 var crlfchecker = require('./external/crlfchecker/crlfchecker.js');
 var path = require('path');