Fix instance_variable#sub for ruby 1.9

In Ruby 1.8, symbols had the #sub method included from Kernel, but this is not present in ruby 1.9 (and later versions). 

I feel that this is a fairly simple change to get this gem working with ruby 1.9, but let me know if you disagree. I looked briefly at adding a unit test for this, but it was a bit of work to mock up the various Mapping class methods (#name2elename and #_obj2soap, in particular) to test that this does not blow up on modern versions of ruby.