Fix the build task not being copied

Forgot that since I moved the build.js file out of the Jekyll
subgenerator I needed to compose with the new gulp subgenerator, so the
generator was broken. Whoops. Closes #147.

Author: sondr3

generators/app/index.js

@@ -162,6 +162,12 @@ on Github](https://github.com/sondr3/generator-jekyllized).
       local: require.resolve('generator-statisk/generators/gulp')
     });
 
+    this.composeWith('jekyllized:gulp', {
+      options: {}
+    }, {
+      local: require.resolve('../gulp')
+    });
+
     this.composeWith('jekyllized:jekyll', {
       options: {
         projectName: this.props.projectName,