Fixed bug where dist folder is created before placeholder page

If dist folder did not already exist, the folder is created prior to creation of the placeholder page. Without this, the dapp files are never built.

lib/pipeline/pipeline.js

@@ -31,6 +31,7 @@ class Pipeline {
     async.waterfall([
       function createPlaceholderPage(next){
         self.events.request('embark-building-placeholder', (html) => {
+          fs.mkdirpSync(self.buildDir); // create dist/ folder if not already exists
           fs.writeFile(self.buildDir + 'index.html', html, next);
         });
       },