Use Cypt\Utils::compareStrings() in Zend\Auth

- Replace the duplicate _secureStringCompare() methods from the Http
  and Digest Zend\Authentication adapters with
  Zend\Crypt\Utils::compareStrings().
- Update Zend\Authentication's composer.json to reflect the dependency
  on Zend\Crypt.
- This potentially reduces the risk of a possible Information Disclosure
  vulnerability, where the length of the target string could be
  discovered via a timing attack. At least in the Digest adapter, there
  was no security issue before this patch, as the string lengths for
  both strings are _always_ 32 character MD5 hashes, thus there is no
  unknown string length to disclose. I'm not 100% sure about the Http
  adapter.

Potential vulnerability was responsibly disclosed by @Freeaqingme.