VAPI-3162 build fix

Author: nkuma246

lib/bandwidth-sdk.rb

@@ -235,6 +235,7 @@
 require 'bandwidth-sdk/models/bxml/verbs/play_audio'
 require 'bandwidth-sdk/models/bxml/verbs/record'
 require 'bandwidth-sdk/models/bxml/verbs/redirect'
+require 'bandwidth-sdk/models/bxml/verbs/refer'
 require 'bandwidth-sdk/models/bxml/verbs/resume_recording'
 require 'bandwidth-sdk/models/bxml/verbs/ring'
 require 'bandwidth-sdk/models/bxml/verbs/send_dtmf'