Updated SDWebImage to version 3.6

`pod update` failed with SDWebImage 3.6 and MWPhotoBrowser 1.4.0.

```
[!] Unable to satisfy the following requirements:
- `SDWebImage (= 3.6)` required by `Podfile`- `SDWebImage/Core` required by `SDWebImage (3.6)`- `SDWebImage (~> 3.5.4)` required by `MWPhotoBrowser (HEAD based on 1.4.0)`
```

I've fixed this incompatible issue with below modification.

- Renew `MWPhotoBrowser/Libraries/SDWebImage`
- Update podspec