%edit ObjectDefinedOnAFile works only once

Given somefile.py with:
```
class FooBar:
pass
```

On ipython:
```
In [1]: from somefile import *

In [2]: %edit FooBar
Editing... Waiting for Emacs...
done. Executing edited code...

In [3]: %edit FooBar
/home/eduardo/.virtualenvs/smartth/local/lib/python3.5/site-packages/IPython/core/magics/code.py:500: UserWarning: The file where `` was defined cannot be read or found.
'cannot be read or found.' % data)

```

Using:

```
$ python -c "import IPython; print(IPython.sys_info())"
{'commit_hash': 'd86648c5d',
'commit_source': 'installation',
'default_encoding': 'UTF-8',
'ipython_path': '/home/eduardo/.virtualenvs/smartth/lib/python3.5/site-packages/IPython',
'ipython_version': '6.1.0',
'os_name': 'posix',
'platform': 'Linux-4.4.0-92-generic-x86_64-with-Ubuntu-16.04-xenial',
'sys_executable': '/home/eduardo/.virtualenvs/smartth/bin/python',
'sys_platform': 'linux',
'sys_version': '3.5.2 (default, Nov 17 2016, 17:05:23) \n[GCC 5.4.0 20160609]'}
```