Clarify the behavior of out of order `push` events

[ejholmes]

Under normal circumstances, GitHub will send a push event in the same order that commits are added. So, for example, if you do the following:

$ git commit --allow-empty
$ git rev-parse HEAD
abcd
$ git push
$ git commit --allow-empty
$ git rev-parse HEAD
dcba
$ git push

Then SlashDeploy would first receive a push event for abcd, then a push event for dcba.

But, GitHub's documentation doesn't specify anything about ensuring that the push events are received in the same order that they were committed.

What happens if GitHub sends the push event for dcba before abcd?