fix OSGi problem in AbstractCalendarValidatorFacto

set classloader explicitly in AbstractCalendarValidatorFactory to avoid issues with OSGi.

Similar workaround to f89a263
Should fix #143

src/main/java/net/fortuna/ical4j/validate/AbstractCalendarValidatorFactory.java

@@ -9,7 +9,7 @@ public abstract class AbstractCalendarValidatorFactory {
 
     private static CalendarValidatorFactory instance;
     static {
-        instance = ServiceLoader.load(CalendarValidatorFactory.class).iterator().next();
+        instance = ServiceLoader.load(CalendarValidatorFactory.class, DefaultCalendarValidatorFactory.class.getClassLoader()).iterator().next();
     }
 
     public static CalendarValidatorFactory getInstance() {