drop support for Ruby 1.8.7 and 1.9.2

Also, now tests against Ruby 2.x.  Fixes https://github.com/afeld/magickly/issues/18, https://github.com/afeld/magickly/issues/20, https://github.com/afeld/magickly/issues/29 and https://github.com/afeld/magickly/issues/31.