OpenSSL usage deprecation warnings from Rubocop

[tas50]

Rubocop has a new cop that detects an upcoming deprecation to the OpenSSL gem that's built into Ruby.

The openssl introducing the deprecation:
ruby/openssl#366

The new rubocop rule currently only in master:
rubocop/rubocop#7950

/Users/tsmith/dev/work/opscode-pushy-client/lib/pushy_client/protocol_handler.rb:457:13: W: Lint/DeprecatedOpenSSLConstant: Use OpenSSL::Digest.new('SHA512') instead of OpenSSL::Digest::SHA512.new.
      sha = OpenSSL::Digest::SHA512.new
            ^^^^^^^^^^^^^^^^^^^^^^^^^^^