Add Slovakia to list of 6-digit shortcodes that libphonenumber misses

Fixes #4478
// FREEBIE

src/org/thoughtcrime/securesms/util/ShortCodeUtil.java

@@ -30,7 +30,7 @@ public class ShortCodeUtil {
       String                  bareNumber        = number.replaceAll("[^0-9+]", "");
 
       // libphonenumber doesn't seem to be correct for Germany and Finland
-      if (bareNumber.length() <= 6 && ("DE".equals(localCountryCode) || "FI".equals(localCountryCode))) {
+      if (bareNumber.length() <= 6 && ("DE".equals(localCountryCode) || "FI".equals(localCountryCode) || "SK".equals(localCountryCode))) {
         return true;
       }