Precedence on rules not working

I'm trying to use precedence on production rules and it doesn't seem to be working.

I'm using the same grammar of simple mathematical expressions used in [the docs](https://rply.readthedocs.io/en/latest/users-guide/parsers.html). All I've done is copy-and-paste the lexer, ast, parser and implicit_multiplication code from docs into a file.

The result of that is:

```
>>> parser.parse(lexer.lex('1 + 2 * 3')).eval()
7
>>> parser.parse(lexer.lex('1 + 2 3')).eval()
9
```

This is on python 3.6.1 with rply 0.7.4