Replace nil signature with empty string.

Hello,

I found a bug where Rack will return an HTTP 500 error when the X-Twilio-Signature header is nil or missing instead of an HTTP 403. The following exception was returned.

```
NoMethodError: undefined method `bytesize' for nil:NilClass
        /Users/ryan/.gem/ruby/2.1.5/gems/twilio-ruby-3.14.1/lib/twilio-ruby/util/request_validator.rb:27:in `secure_compare'
        /Users/ryan/.gem/ruby/2.1.5/gems/twilio-ruby-3.14.1/lib/twilio-ruby/util/request_validator.rb:12:in `validate'
        /Users/ryan/.gem/ruby/2.1.5/gems/twilio-ruby-3.14.1/lib/rack/twilio_webhook_authentication.rb:33:in `call'
        /Users/ryan/.gem/ruby/2.1.5/gems/rack-1.5.2/lib/rack/lint.rb:49:in `_call'
        /Users/ryan/.gem/ruby/2.1.5/gems/rack-1.5.2/lib/rack/lint.rb:37:in `call'
        /Users/ryan/.gem/ruby/2.1.5/gems/rack-1.5.2/lib/rack/showexceptions.rb:24:in `call'
        /Users/ryan/.gem/ruby/2.1.5/gems/rack-1.5.2/lib/rack/commonlogger.rb:33:in `call'
```

I have included a pull request that replaces the signature with an empty string if ```env['HTTP_X_TWILIO_SIGNATURE']``` is nil. Thank you.