Use OpenSSL::Digest instead of OpenSSL::Digest::Digest

Since Ruby 2.1.0 the OpenSSL library has deprecated OpenSSL::Digest::Digest.  Using it produces a warning.  It should use OpenSSL::Digest instead.

http://www.ruby-doc.org/stdlib-2.1.0/libdoc/openssl/rdoc/OpenSSL/Digest/Digest.html