use OpenSSL::Digest instead of OpenSSL::Digest::Digest which is deprecated as of Ruby 2.1.0 and working since 1.8.7
Explicitly require mime-types < 2.0.0 if run for ruby versions < 1.9

rest-client stopped supporting Ruby 1.8 for quite a while and so does
the mime-types gem, which fails to install on 1.8 for newer versions.

Thanks @pekeler for the hint & commit!
Also test on 2.1.x