Add support for Twilio voice verification.

src/main/java/org/whispersystems/textsecuregcm/configuration/TwilioConfiguration.java

@@ -33,6 +33,10 @@ public class TwilioConfiguration {
   @JsonProperty
   private String number;
 
+  @NotEmpty
+  @JsonProperty
+  private String localDomain;
+
   public String getAccountId() {
     return accountId;
   }
@@ -44,4 +48,8 @@ public class TwilioConfiguration {
   public String getNumber() {
     return number;
   }
+
+  public String getLocalDomain() {
+    return localDomain;
+  }
 }