MDL-67953 grunt: Spawn new process for gherkinlint

Gherkinlint stores the list of processed features for duplicate
detection in an Object which is statically defined.
This causes it to bleed between runs.

To alleviate this we must spawn a new process.

Prior to this patchset we were calling a new Gherkinlint process which
had the same effect anyway.

Gruntfile.js

@@ -151,6 +151,9 @@ module.exports = function(grunt) {
                 tasks: ['yui']
             },
             gherkinlint: {
+                options: {
+                    nospawn: false,
+                },
                 files: ['**/tests/behat/*.feature'],
                 tasks: ['gherkinlint']
             }