Fix SSL certificate verification

In ruby 1.9.3, exceptions in Net::HTTP#verify_callback are ignored

I was seeing messages like this:
```ruby/1.9.1/net/http.rb:799: warning: exception in verify_callback is ignored```

and this test was failing:

```
  1) RestClient::Request ssl verification is successful with the correct ca_file
     Failure/Error: expect { request.execute }.to_not raise_error
       expected no Exception, got #<OpenSSL::SSL::SSLError: SSL_connect returned=1 errno=0 state=SSLv3 read server certificate B: certificate verify failed> with backtrace:
         # ./lib/restclient/request.rb:179:in `transmit'
         # ./lib/restclient/request.rb:68:in `execute'
         # ./spec/integration/request_spec.rb:12:in `block (4 levels) in <top (required)>'
         # ./spec/integration/request_spec.rb:12:in `block (3 levels) in <top (required)>'
     # ./spec/integration/request_spec.rb:12:in `block (3 levels) in <top (required)>'
```