Check that key exists on branch

[CJ-Wright]

I have a branch which did not have the deploy key file and everything passed. When it merged it failed because it couldn't find the key. Would it be possible to fail if the key doesn't exist?

[asmeurer]

We need to see if there is enough information to differentiate between the key not being there because it was omitted and it not being there because Travis provide have access to it. I think it's possible. We just need to be careful to avoid false positive errors for things like when Travis runs on a fork.