Make NamespaceFilter.ISPN_NS not rely on hardcoded version

From Vladimir:

"I coded this part and I thought it was an accident waiting to happen. However, I could not use expression "urn:infinispan:config:" + Version.getMajorVersion() because compiler forces us to use constant expressions [1] here.

If you agree we can add another public static field in Version.java:
public static final String MAJOR_VERSION = "4.2";

Maybe we have similar use cases somewhere else as well!

[1] http://java.sun.com/docs/books/jls/third_edition/html/expressions.html#15.28"