Fix deprecated fastGetInstanceVariable calls.
Fix deprecated fastGetInstanceVariable calls.

This removes warnings when compiling against JRuby 1.7.

mwiley/jruby-sandbox,Telmate/jruby-sandbox,k-yamada/jruby-safe,k-yamada/jruby-safe,omghax/jruby-sandbox,omghax/jruby-sandbox,mwiley/jruby-sandbox,Telmate/jruby-sandbox