upgraded javassist to 3.23.1-GA to suppress warning with JDK9+

[tomparle]

In addition to #1245 that fix JDK9+ compatibility (see #1200), upgrade javassist to latest version which is JDK9 compatible to suppress this warning :
WARNING: Illegal reflective access by javassist.util.proxy.SecurityActions (file:.../play-1.x/framework/lib/javassist-3.21.0-GA.jar) to method java.lang.ClassLoader.defineClass(java.lang.String,byte[],int,int,java.security.ProtectionDomain)

[asolntsev]

Approved (if Travis build becomes green).

[tomparle]

Thanks Andrei.
My bad not to have seen some test failed, I'm going to check and fix this as soon as possible !

[tomparle]

OK, it took me more time than expected to understand the issue and finally fix it, but I'm glad to have found it out !
The class PropertiesEnhancer seems to have contained a workaround to fix an issue in javassist previous version :
// protected classes will be considered public by this call && Modifier.isPublic(ctField.getDeclaringClass().getModifiers());

This call to Modifier.isPublic() should have returned false for a protected class (from javassist flags), but was returning true up to javassist version 3.21, hence the previous workaround in Play with the previous comment.
Starting from Javassist 3.23, the method Modifier.isPublic() correctly returns false for a protected class, and returns true for the method Modifier.isProtected(), hence the proposed fix.

[xael-fry]

All is green.
Merge it master
Thanks @asolntsev @tomparle