Tweak how we determine build status

Build's config can have key-value pair `:matrix => nil` under some
circumstances.

If this happens, `[]` will throw a NoMethodError. We need to protect
ourselves.

Ideally, we want to write

```
matrix = Hash(config.fetch(:matrix, {}))
if matrix[:fast_finish]
⋮
```

but `Kernel.Hash` is not available in 1.9.3.