Added missing visitlong()

Using meta 0.4.1, the following code throws an exception about missing `visitlong()`:

```python
import meta

def f(x):
	return x & 0xffff0000

f_ast = meta.decompiler.decompile_func(f)
meta.asttools.print_ast(f_ast)
print meta.dump_python_source(f_ast)
```

The exception is:
```python
AttributeError: 'ExprSourceGen' object has no attribute 'visitlong'
```

The proposed patch fixes this.
I am not sure why `decompile_func()` would generate a `long` type as a node in the first place. This might hide another issue.