Stringify platform sent to bundler webhook

This addresses bundler/bundler-api#35 and bundler/bundler-api#36

The payload used to be [an interpolated string](https://github.com/rubygems/rubygems.org/commit/0739e08#L0L71). Several months ago it was converted to a Hash and `#to_json`'d. This creates the following payload:

```
{
  "name"=>"kafka-jars",
  "version"=>"0.8.0.pre1",
  "platform"=>{
    "cpu"=>nil,
    "os"=>"java",
    "version"=>nil
  },
  "prerelease"=>true,
  "rubygems_token"=>"tokenmeaway"
} 
```

With this patch, the payload is now corrected:

```
{
  "name"=>"kafka-jars",
  "version"=>"0.8.0.pre1",
  "platform"=>'java',
  "prerelease"=>true,
  "rubygems_token"=>"tokenmeaway"
} 
```