FIX start and count of urls is ignored

When setting the start and count values for the `RebuildStaticCacheTask` the limits are ignored because the $urls variable is never reused after it is defined.
FIX Config settings returned to normal after publishing completes

When calling this function onAfterWrite of a DO, the site theme was used to render the CMS form. This fix corrects that by restoring all the config options that are changed
Merge pull request #33 from dhensby/patch-2

FIX start and count of urls is ignored
Fix incorrect set call.
Running CI against SilverStripe master
Merge pull request #21 from dhensby/patch-1

Use Config::nest() where appropriate