Expression with negative value failed to compile

spel2js failed to compile expression with negative value
version: 0.2.6

```javascript
test('test', () => {
    let c = spel2js.SpelExpressionEvaluator.compile('1.0 > -5000.0')  // fail to compile
    console.log(c);
})

// test failed
// Cannot read property 'setParent' of undefined

```

