Gruntfile.js: Update filter for non-existing files to use OS-agnostic path-handling.
Gruntfile.js: Update filter for non-existing files to use OS-agnostic path-handling.

arschmitz/uglymongrel-jquery-mobile,arschmitz/uglymongrel-jquery-mobile,hinaloe/jqm-demo-ja,hinaloe/jqm-demo-ja,arschmitz/jquery-mobile,arschmitz/uglymongrel-jquery-mobile,arschmitz/jquery-mobile,hinaloe/jqm-demo-ja,arschmitz/jquery-mobile