rename repository to grunt-yoast-tasks

[dutchmartin]

The name plugin-grunt-tasks is to generic to upload to npm, so I propose to change it and remove all references to the old name.
Other names could be decided based on what this plug-in should contain, for the current plug-n stase, it could be named grunt-version-management
If the plug-in will only contain our custom tasks, then we should name it grunt-yoast-custom-tasks or grunt-yoast-tasks for a shorter name