{
	"name": "firesphere/silverstripe-newsmodule",
	"description": "A ModelAdmin based newsmodule to prevent clutter in the SiteTree",
	"type": "silverstripe-module",
	"keywords": ["silverstripe", "news", "blog", "slideshow"],
	"authors": [
	{
		"name": "Simon 'Sphere' Erkelens",
		"email": "simon@casa-laguna.net"
	}
	],
	"support": {
		"issues": "https://github.com/Firesphere/silverstripe-newsmodule/issues"
	},
	"require": {
		"silverstripe/framework": "3.*",
		"composer/installers": "*",
		"colymba/GridFieldBulkEditingTools" : "*",
		"UndefinedOffset/SortableGridField" : "*"
	},
	"extra": {
		"installer-name": "silverstripe-newsmodule"
	}
}
{
	"name": "firesphere/newsmodule",
	"description": "A ModelAdmin based newsmodule to prevent clutter in the SiteTree",
	"type": "silverstripe-module",
	"keywords": ["silverstripe", "news", "blog", "slideshow"],
	"authors": [
	{
		"name": "Simon 'Sphere' Erkelens",
		"email": "simon@casa-laguna.net"
	}
	],
	"support": {
		"issues": "https://github.com/Firesphere/silverstripe-newsmodule/issues"
	},
	"require": {
		"silverstripe/framework": "3.*",
		"composer/installers": "*",
		"colymba/GridFieldBulkEditingTools" : "*",
		"UndefinedOffset/SortableGridField" : "*"
	},
	"extra": {
		"installer-name": "silverstripe-newsmodule"
	}
}

Remove 'silverstripe-' from package name
Remove 'silverstripe-' from package name

Otherwise it will check out into the 'silverstripe-newsmodule' directory on installation.

The fact that type = silverstripe-module will mean that it will still be picked up by http://extensions.andrewshort.name/ and eventually http://extensions.silverstripe.org/