Update tests to new HSpec API. Fixes #12, fixes #13.

This does not include the change to the cabal file, because that
would increase likelihood of users entering cabal hell. If the hspec
version is too low, then only a warning will be issued.

(cherry picked from commit e0a32457d62e61e052ca460b98bbc886ca45c9b1)
Remove -Werror from running the testsuite.

-Werror is still enabled on Travis, as it should be.