Remove unnecessary tests.

I was being over zealous with testing previously.
Fix for 1.9, 2.0, etc.

Drop PBKDF2, use OpenSSL built in PBKDF2 checks instead.

In addition, use an eql_time_cmp check which protects against timing
attacks.

More info here:

    http://www.ruby-doc.org/stdlib-2.0/libdoc/openssl/rdoc/OpenSSL/PKCS5.html