Error running code samples in README

Tried running a couple of the code samples in the Parsing section, got the same error each time:

```
In lib\ebnf\src\ebnf\grammar\rule.cr:89:48

 89 | def initialize(@atoms = Array(Atom).new, @action = nil)
                                               ^
Error: instance variable @action of EBNF::Bison::Rule was inferred to be Nil, but Nil alone provides no information
```