avoid 'invalid byte sequence in UTF-8'

when Graph API server responses with invalid UTF-8 string, fb_graph's node#handle_response method raise error.

```
invalid byte sequence in UTF-8
fb_graph (2.7.8) lib/fb_graph/node.rb:138:in `handle_response'
```

to avoid this problem, strip invalid string from response body.
use [String#scrub method](https://github.com/ruby/ruby/blob/1e8a05c1dfee94db9b6b825097e1d192ad32930a/string.c#L7770-L7783) introduced in Ruby 2.1, older version use [backport of this method](https://github.com/hsbt/string-scrub) or [known workaround](http://stackoverflow.com/questions/14035307/ruby-invalid-byte-sequence-in-utf-8-argumenterror)