support ObjectExpression properties without a `type` member

Fixes #4. Sorry, no tests since there's no test suite. Here's what I was using to test it while developing:

```javascript
estraverse = require('./estraverse.js')                                                                                      
esprima = require('esprima');

ast = esprima.parse('c = {a: b}');
delete ast.body[0].expression.right.properties[0].type;

estraverse.traverse(ast, {enter: function(node){ console.dir(node); }});
estraverse.replace(ast, {enter: function(node){ console.dir(node); return node; }});
```