tools/push-n-tag-release.sh: fix bashisms

voodoo@hestia:/opt/github/stable-kernel$ ./checkbashisms tools/push-n-tag-release.sh
script tools/push-n-tag-release.sh does not appear to be a /bin/sh script
possible bashism in tools/push-n-tag-release.sh line 29 (should be '.', not 'source'):
        source ${DIR}/version.sh

Signed-off-by: Robert Nelson <robertcnelson@gmail.com>