Update fusefs Ruby/C extensions to be Ruby 1.9 compatible

Now that Ruby 1.8.7 (and thus 1.8.x) has been [retired](http://www.ruby-lang.org/en/news/2013/06/30/we-retire-1-8-7/) officially, I am submitted a pull request to remove 1.9+ Ruby/C extension incompatibility. Removing `STR2CSTR` and replacing with the 1.9.x+ `StringValuePtr`.