get_version.sh: Remove - characters from release string

Replace - with . in release strings to fix the following build
error in the dist Makefile target:

  error: line 4: Illegal char '-' in: Release: 4-g1d44b2a
  make: *** [rpms] Error 1

Signed-off-by: Peter Oberparleiter <oberpar@linux.vnet.ibm.com>