Fix -isEqualToString: crashing because of number IDs

Partially reverts 258f3c196536317c2b375f46284c1896c3434c0c.

/cc @alanjrogers
Try to migrate super old numeric objectIDs to strings
Bump model version, migrate accidental numeric objectIDs
Merge branch 'octokit-master' into fix-objectid-comparison
Revert "Try to migrate super old numeric objectIDs to strings"

This reverts commit a19d2bb52db5be52af04241109f8d95ecbe704a3.
Revert "Bump model version, migrate accidental numeric objectIDs"

This reverts commit 25db8d235010dfd6b392880857c4a536dea23cfa.
Simplify everything by just validating objectID
Merge pull request #162 from octokit/fix-objectid-comparison

Fix objectID comparison and migration