bootstrap.sh bug: if bootstrap pulls a new bootstrap, syncing continues with old bootstrap -> bad things happen

Previously, if bootstrap.sh had been changed in the origin repo,
the syncing would continue using the old bootstrap.sh. This can
cause problems such as the following: Say you have added a new
LICENSE file and an appropriate exclusion in the rsync of bootstraph.sh.
When a user does `source bootstrap.sh` the new LICENSE file and
bootstrap.sh file get pulled but the syncing continues with the old
bootstrap which doesn't have the exclusion, thus giving you a LICENSE
file in your ~. This commit gives a basic solution of exiting if the
bootstrap.sh file has changed after the `git pull`.

Ideally, this could be automated because there isn't any real reason the user should be bothered. I just wasn't sure how to implement it.