fixed encoding issue

I got an exception running 'fontello convert':

gems/fontello_rails_converter-0.2.0/lib/fontello_rails_converter/cli.rb:76:in `gsub!': invalid byte sequence in UTF-8 (ArgumentError)

found this fix on: http://robots.thoughtbot.com/fight-back-utf-8-invalid-byte-sequences/
Merge pull request #11 from hqm42/patch-1

fixed utf-8 issue