Add test for sequencing HList of ValidationNels

There was previously a test for this case, but it was removed when Scala
2.11 introduced a regression that caused it to break. It doesn't work if
the `ValidationNel` type alias is used, but it works if the full type
(`Validation[NonEmptyList[A], B]`) is used, so I'm adding a test for
that case. See #20 for more background.
Merge pull request #22 from ceedubs/sequence-validationnel-test

Add test for sequencing HList of ValidationNels