chore(build.config): remove 'fs' module

I'm not sure why <code>var fs = require('fs');</code> is here anymore

Closes #870

config/build.config.js

@@ -1,4 +1,3 @@
-var fs = require('fs');
 module.exports = {
   dist: 'dist',
   distJs: 'dist/js',

gulpfile.js

@@ -132,7 +132,7 @@ gulp.task('changelog', function(done) {
     version: pkg.version,
   }, function(err, data) {
     if (err) return done(err);
-    require('fs').writeFileSync('CHANGELOG.md', data);
+    fs.writeFileSync('CHANGELOG.md', data);
     done();
   });
 });