[less] update grunt-contrib-less v3.3.0

Upgraded [v3.3.0] otherwise::

`  width: calc(100% - 5rem);`

becomes `width: 95%` once compiled by less version 1.4.1.

[v3.3.0] https://github.com/gruntjs/grunt-contrib-less/releases/tag/v3.0.0

Suggested-by: @dalf in commit 1204e4f0
Signed-off-by: Markus Heiser <markus.heiser@darmarit.de>

searx/static/themes/simple/package.json

@@ -4,7 +4,7 @@
     "grunt-contrib-concat": "~1.0.1",
     "grunt-contrib-cssmin": "^2.2.1",
     "grunt-contrib-jshint": "~1.1.0",
-    "grunt-contrib-less": "^1.4.1",
+    "grunt-contrib-less": "~3.0.0",
     "grunt-contrib-uglify": "~3.4.0",
     "grunt-contrib-watch": "~1.1.0",
     "grunt-webfont": "^1.7.1",