fix(tag_release): Add a few sanity checks.

- it shouldn't be possible to set the SDK version to the same as the new
  tag's version. The SDK must always be a previous build.
- don't fail if there isn't any old manifests to git rm.

There are ways to improve, it'd be sexy if it was truly safe and we
could throw around annoying terms like idempotent to make this kind of
sequence just work but doesn't yet: tag_release; tag_release --push
Merge pull request #141 from marineam/tag-checks

fix(tag_release): Add a few sanity checks.