Ripper is not returning unary minus with proper symbol


```text
jruby -rripper -e 'p Ripper.sexp_raw("-7")'
[:program, [:stmts_add, [:stmts_new], [:unary, :-, [:@int, "7", [1, 1]]]]]
```

This should be:

```text
1184% ruby -rripper -e 'p Ripper.sexp_raw("-7")'
[:program, [:stmts_add, [:stmts_new], [:unary, :-@, [:@int, "7", [1, 1]]]]]
```