Update Gruntfile to use 'sails-linker' task name used when registering a...

...ctual task

The task defined in tasks/scriptlinker.js registers itself as "sails-linker":

```
grunt.registerMultiTask('sails-linker', 'Your task description goes here.', function() {
	// Merge task-specific and/or target-specific options with these defaults.
	var options = this.options({
		startTag: '<!--SCRIPTS-->',
		endTag: '<!--SCRIPTS END-->',
		fileTmpl: '<script src="%s"></script>',
		appRoot: '',
		relative: false
	});
...
```
However, the `test` task in the Gruntfile looks for a 'scriptlinker' task: 
```
grunt.registerTask('test', ['clean', 'scriptlinker:default_options', 'nodeunit']);
```

This patch updates the Gruntfile to use the task name 'sails-linker' instead of 'scriptlinker' when defining default options and in the test task definition.