Fix parsing of boolean expressions

The following program leads to an error during parsing:

```
foo = function() -> Boolean {
    a = 20
    b = 19
    return a >= b
}
```

```
[ERROR, 20:47:27]: Encountered syntax error.
|    foo = function() -> Boolean {
|        a = 20
|        b = 19
|        return a >= b
                   ~
Unable to consume token: =
|    }
|    
```