drop dependency on deprecated ruby-hmac gem, fixes #2034

Ruby 1.8 compiled against OpenSSL 0.9.8 supports SHA256 for some time now.
Ref: https://github.com/ruby/ruby/blob/ruby_1_8_7/ChangeLog#L12025-L12029
Merge pull request #2513 from miketheman/2034_remove_dep_ruby_hmac

drop dependency on deprecated ruby-hmac gem, fixes #2034