Fix travis builds

`Nokogiri >= 1.6` and `mime-types >= 2.0` are incompatible with `ruby 1.8.7`.

If not pinned to a specific version, even when building with `ruby 1.8.7`, bundler will attempt to install the most recent version of both gems, resulting in an error.

This PR pins `nokogiri` and `mime-types` to the latest supported versions when using `ruby 1.8.7`.