Allow to test PR on synchronize

From commit message:

```
When new commit is pushed to a target branch, github sends us
synchronize event. We haven't tested this commit, unless head was
changed, but it is a valid use case - when master changes, PR may no
longer pass, so it's good to retest it.

This commit allows to enable such a behavior for a specific repository
slug.
```

/cc @rkh