Remove usage of javax.xml.bind.DatatypeConverter (removed in Java 9) 

JDK9 marks the package javax.xml.bind as deprecated for removal.

The current use results in a java.lang.NoClassDefFoundError :

`Exception in thread "main" java.lang.NoClassDefFoundError: javax/xml/bind/DatatypeConverter
	at io.jsonwebtoken.impl.Base64Codec.decode(Base64Codec.java:26)
	at io.jsonwebtoken.impl.DefaultJwtBuilder.signWith(DefaultJwtBuilder.java:99)`