Tweak `Build.matrix?` and `Build.matrix_keys_for`

`Build.matrix?` should return `true` if and only if there is a key on which
we should expand, given the specified language.
For example, `Build.matrix? {'perl' => ['5.12', '5.10']}` should not be `true`
without `{'language' => 'perl'}`.

Consequently, modify `Build.matrix_keys_for` specs.
Move the mysterious `:branch` to where it belongs

We need to expand on `:branch` in all cases; but having a
` + [:branch]` seems out of place. Move it to
`EXPANSION_KEYS_UNIVERSAL` for better organization.