Disallow alpha chars in country code field

// FREEBIe

src/org/thoughtcrime/securesms/RegistrationActivity.java

@@ -298,7 +298,7 @@ public class RegistrationActivity extends BaseActionBarActivity {
   private class CountryCodeChangedListener implements TextWatcher {
     @Override
     public void afterTextChanged(Editable s) {
-      if (TextUtils.isEmpty(s)) {
+      if (TextUtils.isEmpty(s) || !TextUtils.isDigitsOnly(s)) {
         setCountryDisplay(getString(R.string.RegistrationActivity_select_your_country));
         countryFormatter = null;
         return;