[bug] Fix debian_nextcodename on wheezy hosts

I noticed this behaviour because $::debian_nextcodename was
"squeeze" on a wheezy host.
For debugging, i inserted a "puts codenames" in
lib/facter/debian_nextcodename.rb, and it turned out that it
was sorted differently on wheezy and jessie hosts:

On wheezy:

    buster
    stretch
    jessie
    wheezy
    squeeze
    lenny

On jessie:

    lenny
    squeeze
    wheezy
    jessie
    stretch
    buster

So i decided to rewrite this so this doesn't happen again.

lib/facter/debian_nextcodename.rb

@@ -8,11 +8,10 @@ def debian_codename_to_next(codename)
   if codename == "sid"
     return "experimental"
   else
-    codenames = Facter::Util::Debian::CODENAMES.values
-    i = codenames.index(codename)
-    if i and i+1 < codenames.count
-      return codenames[i+1]
-    end
+    codenames = Facter::Util::Debian::CODENAMES
+    versions  = Facter::Util::Debian::CODENAMES.invert
+    current_version = versions[codename]
+    return codenames[(current_version.to_i + 1).to_s]
   end
 end
 

lib/facter/util/ubuntu.rb

@@ -14,6 +14,7 @@ module Facter
         "utopic",
         "vivid",
         "wily",
+        "xenial"
       ]
     end
   end