Revert "Use rack-utf8_sanitizer gem"

This reverts commit 45f25290e2e4a1f33ec5d3df6ecfb40300318ece.

Use of the :ruby_20 platform requires bundler 1.3, which is not
available on all our supported OSes as a package. See
https://github.com/mysociety/alaveteli/issues/1276 for details.