Fix regexp warning on UTF-8 characters

By `String#force_encoding("binary")` in the `unescape()` method, it's easier to avoid the regexp warning:

````
./gems/simple-rss-ef0d5db568fa/lib/simple-rss.rb:155:
    warning: regexp match /.../n against to UTF-8 string
````

Using this fix today, seems to work all right. My test case is to watch a private RSS feed of an organization on GitHub, and marvel at the error's absence. I'm doing something trivial like:

````
uri = "https://github.com/organizations/%s/%s.private.atom?token=%s"
@rss = SimpleRSS.parse(open(uri))
````

against rapid7/metasploit-framwork (with my RSS token)