Add utf8-cleaner gem

If you make a request to Supermarket with invalid utf8 params it will
raise an exception because of a Rails bug explained here:
http://dev.mensfeld.pl/2014/03/rack-argument-error-invalid-byte-sequence-in-utf-8.
The utf8-cleaner gem sanitizes invalid utf8 strings preventing these
exceptions.
Merge pull request #625 from opscode/utf8

Add utf8-cleaner gem