Explicitly cast Pathname in Puppet::Util#replace_file

Puppet::Util#replace_file contains a call to File#rename
where one of the arguments is of type `Pathname`.  The
actual method signature requires two String arguments,
and JRuby does not appear to implicitly cast in this
situation--so all calls to `replace_file` were failing
on JRuby.  This commit simply makes the cast explicit.