Force UTF-8 encoding for data source

My Ruby installation defaults to US-ASCII (apparently). That makes `nanoc compile` throw an exception, because the files in this template contain UTF-8 (which is cool!). 

After manually setting the encoding for the data source to UTF-8, this issue was solved.

I'm not 100% sure whether this should be part of the template, or actually should be considered to be an error in my Ruby installation.