write sourcemaps as separate files

Author: Carreau

gulpfile.js

@@ -15,7 +15,7 @@ gulp.task('css', function () {
     .pipe(rename({
             suffix: '.min'
         }))
-    .pipe(sourcemaps.write())
+    .pipe(sourcemaps.write('./'))
     .pipe(gulp.dest('./jupyter_notebook/static/style'));
 });