[Truffle] Requiring paths with . for current directory doesn't work

```
$ cat a.rb 
require("./b.rb")
$ cat b.rb 
puts("hi")
$ ruby2.1 a.rb                                                                                
hi
$ ../work/jruby/bin/jruby a.rb 
hi
$ ../work/jruby/bin/jruby -X+T -J-server a.rb 
a.rb:1:in `require': cannot load such file -- ./b.rb (LoadError)
        from a.rb:1:in `<main>'
```
