Flatten ignored exceptions.

This part of [readme](https://github.com/MindscapeHQ/raygun4ruby/blob/169e9896c1852de1dbe1f15796292bb0120eafae/README.md#ignoring-some-errors) doesn't work now.

You need to pass exception instead of array.

```ruby
config.ignore  << MyApp::AnExceptionIDontCareAbout
```

To allow multiple exceptions onliner I added flatten when it's accessing ignore array.

```ruby
config.ignore  << [MyApp::AnExceptionIDontCareAbout, MyApp::AnExceptionIDontCareAboutAlso]
```