Change version format

Now, the Rubi version has the format xx.yy.zz.pp.

- xx.yy.zz is the version of the Rubi rules (or Rubi engine as Albert likes
  to call it). This will be increased if Albert is making improvements on
  the integration rules
- pp is the version of the package wrapper code.

Since these two parts are "basically" unrelated, we handle versioning
separately. In this way, Albert can keep his versioning for the rules
and every time some improvements on the package code is done, we increase
the pp number.

Rubi/PacletInfo.m

@@ -5,7 +5,7 @@
 
 Paclet[
   Name -> "Rubi",
-  Version -> "4.17",
+  Version -> "4.15.2.1",
   MathematicaVersion -> "11+",
   Description -> "Package for Rule-based Integration",
   Creator -> "Albert D. Rich",