Remove deprecation warnings when using minitest-rails 0.9 and higher

The patch removes deprecation warnings generated by requiring Draper into
an app that uses minitest-rails 0.9 and above. The changes were performed
according to the minitest [Upgrading to 0.9](https://github.com/blowmage/minitest-rails/wiki/Upgrading-to-0.9)
page.

No new tests are added, but if you upgrade bundle to use minitest-rails 0.9
you'll see that deprecation warnings have disappeard.

Cheers!