Fixed Encoding::InvalidByteSequenceError

Hi there.

Thanks for the great and simple app.

I've found an error with default encoding for ruby processes. I created a new Slim file, added there some strings and when I tried to add russian phrase Prepros started warning me with about unsuccessful compilations, like this one: http://d.pr/i/tfak

I did some investigation and made a simple patch for your gem_loader.rb file.

Here I'm just forcing to use UTF-8 (who needs to use anything else?) as the `default_external` encoding for Ruby (which should be used to parse input files).