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/
Merge pull request #15 from sminnee/patch-1

Newbie git-question. But why?