rb_eql() not implemented in JRuby

(Ref: https://github.com/libyui/ruby-ui/pull/6 )

We noticed rb_eql is not provided by JRuby, so in order to compile our extension we had to replace:

```
return rb_eql(this->rubyValue(), that->rubyValue());
```

with:

```
return rb_funcall(this->rubyValue(), rb_intern("eql?"), 1, that->rubyValue());
```
