Fix upload return type

I tried to use the gem but I got an error after uploading an image.
It tries to call an old `gallery_images` method to create an Image object (I supposed).
Now it creates the object, and specs relative to image upload are green again.

However:
- mind if I commit Gemfile.lock too?
- there's another spec failing, calling a method doesn't exist:
```
  1) images should have comments
     Failure/Error: image.comments.should_not be_empty
     NoMethodError:
       undefined method `comments' for #<Imgur::Client::Image:0x007fc9e4411e18>
     # ./spec/images_spec.rb:14:in `block (2 levels) in <top (required)>'
```
do you agree if I prepare a PR to remove the spec?
- if you accept this PR, can you push it to rubygems?

Thanks