Restore formatting for `all` failure messages.

This changed in c03cdc734b729481cffcb2d30cbf5670b7cfc2e6.
It was a consequence of fixing `Pretty#to_sentence`, but
as it turns out, we don’t even need to use that. It reads
better without the parens, so this restores the old formatting.
Merge pull request #621 from rspec/fix-all-failure-messages

Restore formatting for `all` failure messages.