fix(gen:heroku): allow for grunt or gulp

heroku/index.js

@@ -112,12 +112,13 @@ Generator.prototype.copyProcfile = function copyProcfile() {
   });
 };
 
-Generator.prototype.gruntBuild = function gruntBuild() {
+Generator.prototype.build = function build() {
   if(this.abort) return;
   var done = this.async();
+  var buildCommand = this.filters.grunt ? 'grunt build' : 'gulp build';
 
   this.log(chalk.bold('\nBuilding dist folder, please wait...'));
-  var child = exec('grunt build', function (err, stdout) {
+  var child = exec(buildCommand, function (err, stdout) {
     done();
   }.bind(this));
   child.stdout.on('data', function(data) {
@@ -187,8 +188,12 @@ Generator.prototype.gitForcePush = function gitForcePush() {
         this.log(chalk.green('\nYou may need to address the issues mentioned above and restart the server for the app to work correctly.'));
       }
 
-      this.log(chalk.yellow('After app modification run\n\t' + chalk.bold('grunt build') +
-      '\nThen deploy with\n\t' + chalk.bold('grunt buildcontrol:heroku')));
+      this.log(chalk.yellow(
+        'After app modification run\n\t' + 
+        chalk.bold(this.filters.grunt ? 'grunt build' : 'gulp build') +
+        '\nThen deploy with\n\t' +
+        chalk.bold(this.filters.grunt ? 'grunt buildcontrol:heroku' : 'gulp buildcontrol:heroku')
+      ));
     }
     done();
   }.bind(this));