use rest-client which uses a Ruby 1.9.2 compatible version of mime-types

mime-types which rest-client uses is no longer compatible with Ruby 1.9.2

I issued a PR to rest-client, which supports Ruby 1.9.2, to be more restrictive about the version of the mime-types gem used.

https://github.com/rest-client/rest-client/pull/322

In the meantime this PR pins rest-client v1.6.8 which uses mime-types 1.25.1