JRuby compatibility

[cvigano]

The gem fails to build with JRuby as it uses obsolete and deprecated Config:

 $ JRUBY_OPTS=-Xcext.enabled=true jgem install cbor
Building native extensions.  This could take a while...
ERROR:  Error installing cbor:
        ERROR: Failed to build gem native extension.

    /home/cvigano/.rubies/jruby-1.7.16/bin/jruby extconf.rb
/home/cvigano/.rubies/jruby-1.7.16/lib/ruby/shared/mkmf.rb:14: Use RbConfig instead of obsolete and deprecated Config.
mkmf.rb can't find header files for ruby at /home/cvigano/.rubies/jruby-1.7.16/lib/native/include/ruby/ruby.h


Gem files will remain installed in /home/cvigano/.gem/jruby/1.9.3/gems/cbor-0.5.9.0 for inspection.
Results logged to /home/cvigano/.gem/jruby/1.9.3/gems/cbor-0.5.9.0/ext/cbor/gem_make.out