Raise Intercom::ServiceUnavailableError when endpoint returns 502

Hi, the Gem used to raise this exception when the endpoint would return _502 Bad Gateway_ (which happens from time to time):

```html
JSON::ParserError: 757: unexpected token at '<html> <head><title>502 Bad Gateway</title></head> <body bgcolor="white"> <center><h1>502 Bad Gateway</h1></center> <hr><center>nginx/1.4.2</center> </body> </html> '
```

This change makes it raise `Intercom::ServiceUnavailableError` instead so it can be rescued reliably in our apps.