use default property instead of lazy builder

to prevent empty string in CPAN test.

lib/Weather/Underground/Forecast.pm

@@ -58,7 +58,7 @@ has 'raw_data' => (
 has 'source_URL' => (
     is         => 'ro',
     isa        => 'Any',
-    lazy_build => 1,
+    'default'  => 'http://api.wunderground.com/auto/wui/geo/ForecastXML/index.xml?query=',
 );
 
 # When the location changes, we want to clear the data to insure a new data fetch will happen.
@@ -192,12 +192,6 @@ sub _build_raw_data {
     return $content;
 }
 
-sub _build_source_URL {
-    my $self = shift;
-    return
-      'http://api.wunderground.com/auto/wui/geo/ForecastXML/index.xml?query=';
-}
-
 __PACKAGE__->meta->make_immutable;
 1