Fix input file encoding

[mizunashi-mana]

I got an error such as

/usr/local/bundle/gems/mdl-0.3.1/lib/mdl/doc.rb:31:in `split': invalid byte sequence in US-ASCII (ArgumentError)
    from /usr/local/bundle/gems/mdl-0.3.1/lib/mdl/doc.rb:31:in `initialize'
    from /usr/local/bundle/gems/mdl-0.3.1/lib/mdl/doc.rb:44:in `new'
    from /usr/local/bundle/gems/mdl-0.3.1/lib/mdl/doc.rb:44:in `new_from_file'
    from /usr/local/bundle/gems/mdl-0.3.1/lib/mdl.rb:74:in `block in run'
    from /usr/local/bundle/gems/mdl-0.3.1/lib/mdl.rb:72:in `each'
    from /usr/local/bundle/gems/mdl-0.3.1/lib/mdl.rb:72:in `run'
    from /usr/local/bundle/gems/mdl-0.3.1/bin/mdl:10:in `<top (required)>'
    from /usr/local/bundle/bin/mdl:22:in `load'
    from /usr/local/bundle/bin/mdl:22:in `<main>'

Probably, the reason was LANG=C.
I input UTF-8 file. But, Ruby use US-ASCII from LANG=C, so, String.split try to split "ASCII string".
I fixed this problem by setting env RUBYOPT=-EUTF-8.

However, I suggest guessing file encoding.

Fix: https://github.com/mivok/markdownlint/blob/master/lib/mdl/doc.rb#L44

- self.new(File.read(filename))
+ self.new(Kconv.toutf8(File.read(filename)))

using http://docs.ruby-lang.org/en/2.3.0/Kconv.html#method-c-toutf8