Bump mime-types requirement to ~> 2.0.

Mime-types uses a modified semantic versioning scheme MAJOR.MINOR where
major release 2.0 dropped compatibility for Ruby 1.8. Given that
rest-client 1.7.x doesn't support Ruby 1.8 either, we can safely
upgrade.

The 1.6-legacy branch will maintain support for Ruby 1.8 and mime-types
~> 1.x.

Also switch to the `~>` operator for the dependency so we don't
accidentally upgrade to new major releases.

Fixes: #235