Fix: JAXB NoClassDefFoundError on Java 9+ (#218)

JAXB APIs are considered to be a part of Java EE and was removed by default in Java SE 9.
See: https://stackoverflow.com/a/47412779/6238618

src/com/sheepit/client/Server.java

@@ -340,7 +340,7 @@ public class Server extends Thread implements HostnameVerifier, X509TrustManager
 				
 				script += jobData.getRenderTask().getScript();
 
-				String validationUrl = URLDecoder.decode(jobData.getRenderTask().getValidationUrl());
+				String validationUrl = URLDecoder.decode(jobData.getRenderTask().getValidationUrl(), "UTF-8");
 
 				Job a_job = new Job(
 						this.user_config,