Faroese phone numbers are 6 digits in length and has

a 3-digit dialing code, but the validation regex
 requires 10-digit numbers.

Validates numbers the same way libsignal-service-java does it.

Closes #72

src/main/java/org/whispersystems/textsecuregcm/util/Util.java

@@ -43,7 +43,11 @@ public class Util {
   }
 
   public static boolean isValidNumber(String number) {
-    return number.matches("^\\+[0-9]{10,}");
+    return number.matches("^\\+[0-9]{10,}")  ||
+           number.matches("^\\+298[0-9]{6}") ||
+           number.matches("^\\+240[0-9]{6}") ||
+           number.matches("^\\+687[0-9]{6}") ||
+           number.matches("^\\+689[0-9]{6}");
   }
 
   public static String encodeFormParams(Map<String, String> params) {