Slightly trim Ruby 2.2.0 release post (en)

@hsbt @nurse

The release post states that:

```
Recent developments mentioned on the Rails blog suggest that Rails 5.0 will take
advantage of Incremental GC as well as Symbol GC.
```

I couldn't find any mention of Incremental GC on the Rails blog, especially not in the linked Rails 4.2. release post. And Symbol GC is already mentioned in the previous paragraph. So please consider to

1. remove this sentence (see this PR)
2. or provide a better reference