Deployment target for Rollbar changed to iOS 12.0

After update I'm getting this warning while archiving my project

```
ld: warning: dylib (../Rollbar.framework/Rollbar) was built for newer iOS version (12.0) than being linked (11.0)
```

I think this commit https://github.com/rollbar/rollbar-ios/commit/2949df4645aea778096394095c0a81381f013935 somehow changed it. I'm really sure you wanted to drop only iOS 8 support 