Fixing issue #4902.
closes #4902, Added a check for complex numbers.

@alefnula Can you add a test if you aren't doing that already? Thanks :)
 release notes as well!
 @alefnula just fyi those are in `doc/source/release.rst`
 Uh sorry, I'm not that familiar with the pandas project workflow yet. Can someone point me what should I add to release notes, and in which test suite/test file should the test for `pandas.core.nanops._ensure_numeric` go? The function is also private for the module and I don't see it used anywhere else, so should I test it directly or using just the public api.

I'm really sorry for this many questions. :-/
 @alefnula No problem.
1. As I said above, release notes are in `pandas/doc/source/release.rst`
2. Test it on something similar to the SO question, e.g., create a heterogeneously typed `DataFrame` and compute the mean. Something like,

``` python
def test_complex_mean(self):
    df = DataFrame({'a': tm.choice(['a', 'b'], size=10), 'b': rand(10).astype(complex) + randn(10) * 1j})
    result = df.mean().item()
    expected = df['b'].values.mean()
    self.assertEqual(result, expected)
```
 @cpcloud OK got it. One more question. `test_series.py` - is this an appropriate place for this test? Since this happens on every aggregate operation on a Series object.
 @alefnula yes...can put it there....try to find a similar group of tests and try it (note that make sure its NOT under SafeForSparse), because I don't think this works on sparse
 @jreback I just found a `test_ensure_int32` int `test_common.py`. Maybe this is a better place for `test_ensure_numeric` since it's testing the exact change?
 @alefnula that's fine too
 @alefnula do you have travis hookup up? see contributing.md for instructions...
 Added tests, updated the `release.rst`.
 @jreback Enabled travis. But saw your post too late, already pushed the changes. :-/
 go ahead and set up travis, then

`git commit --amend -C HEAD`
will trigger travis with a new commit hash
 @jreback Succeeded :) Than you and @cpcloud for assistance. :)
 looks good....pls rebase and squash to 1 commit and should be mergeable
 no need to close ....
 @cpcloud I'm haven't closed it. Github automatically closed it when I rebased and pushed.
 strange
 yeah, that's not supposed to happen at all. very weird.
 wonder if the commit message has naything to do with it?
 It happened two times... When I did the amend and when I rebased :-/
 that's very weird since "fixing" is not [one of the words that closes an issue](https://help.github.com/articles/closing-issues-via-commit-messages)
 thanks!
