Use download.cyanogenmod.org instead of get.cm

The get.cm domain is cool, but to keep things consistent, we should
always point to download.cyanogenmod.org.  get.cm will soon be
redirecting to download.cyanogenmod.org.  Right now both of them work.

Change-Id: I36666ec7a7754363e36f0aef9359620e94c09542