Fix cache to avoid encoding exception.

[marcandre]

Not too sure how come the CI sometimes gets an encoding error:

RuboCop::ResultCache#save when the default internal encoding is UTF-8 writes non UTF-8 encodable data to file with no exception
     Failure/Error:
       rubocop_extra_features
         .select { |path| File.file?(path) }
         .sort!
         .each { |path| digest << Zlib.crc32(IO.read(path)).to_s } # mtime not reliable

     Encoding::InvalidByteSequenceError:
       "\xCE" on US-ASCII
     # ./lib/rubocop/result_cache.rb:179:in `read'
     # ./lib/rubocop/result_cache.rb:179:in `block in rubocop_checksum'
     # ./lib/rubocop/result_cache.rb:179:in `each'
     # ./lib/rubocop/result_cache.rb:179:in `rubocop_checksum'
     # ./lib/rubocop/result_cache.rb:94:in `initialize'
     # ./spec/rubocop/result_cache_spec.rb:7:in `new'