[fix] Firefox compatible (good bye native ES6)

site/public/_gulp/gulpfile.js

@@ -6,6 +6,7 @@ var gulp            = require( 'gulp' ),
     plumber         = require( 'gulp-plumber' ),
     gulp_rename     = require( 'gulp-rename' ),
     gulp_stylus     = require( 'gulp-stylus' ),
+    nib             = require( 'nib' ),
     browser_sync    = require( 'browser-sync' ).create();
 
 var path = '../';
@@ -41,6 +42,7 @@ gulp.task( 'stylesheet', function()
         .pipe( gulp_sourcemaps.init() )
         .pipe( plumber() )
         .pipe( gulp_stylus( {
+            use     : nib(),
             compress: true,
         } ) )
         .pipe( gulp_sourcemaps.write( '.' ) )

site/public/_gulp/package.json

@@ -14,6 +14,7 @@
     "gulp-sourcemaps": "^1.6.0",
     "gulp-stylus": "^2.5.0",
     "gulp-uglify": "^1.1.0",
-    "gulp-watch": "^3.0.0"
+    "gulp-watch": "^3.0.0",
+    "nib": "^1.1.2"
   }
 }