Change package-id

Package-id should be just mathematica; it had been changed to
mathematica9. Please don't change extant package-ids as it will cause
all sorts of problems

mathematica.xml

 <?xml version="1.0" encoding="UTF-8"?>
 <packages>
-<package id="mathematica9" name="Mathematica" revision="%version%.1" reboot="false" priority="30">
+<package id="mathematica" name="Mathematica" revision="%version%.1" reboot="false" priority="30">
  
   <variable name="version" value="9.0.1" />