Change Instagram.respond_to? signature

Ruby 1.9 and 2.0 allow `Object#respond_to?` to receive [two arguments](http://ruby-doc.org/core-2.1.2/Object.html#method-i-respond_to-3F):

```ruby
respond_to?(symbol, include_all=false)
```

Current signature with only one argument makes trouble when using rspec 3.0 to mock `Instagram` class behavior.