Workaround FTBFS in minimal chroot

Building itsalltext currently fails with Ruby 1.9 and 2.0 with the following error:

rake aborted!
invalid byte sequence in US-ASCII
…/Rakefile:51:in `block in load_dtd_entities'
…/Rakefile:50:in `map'
…/Rakefile:50:in `load_dtd_entities'
…/Rakefile:56:in `find_dtd_errors'
…/Rakefile:103:in `block in diff_locale'
…/Rakefile:94:in `each'
…/Rakefile:94:in `diff_locale'
…/Rakefile:41:in `block (2 levels) in <top (required)>'
…/Rakefile:41:in `each'
…/Rakefile:41:in `block in <top (required)>'

Bug-Debian: https://bugs.debian.org/739774