BUG: maintain order in excluding categories
should fix current travis fails on master branch.

i cannot add tests right now because `test_remove_categories` in `test_categorical.py` does not fail on my machine. it also passes on a number of travis builds, and fails only on a couple.

[all green](https://travis-ci.org/pydata/pandas/builds/36359682) except for one IO error (`URLError`) unrelated to this patch.
 well I already have a patch for this with some other code.

thanks anyhow
 > thanks anyhow

@jreback thanks anyhow to you,

even if you have a patch for this shouldn't you respect the time spent for the PR?

I mean if i had seen that someone has already submitted a PR for this, that was a different story. but if i spend couple of hours on this but then someone just come and close it because "i have my own patch for this" that is just annoying!
 @behzadnouri well I did this a couple of days ago, but was out for a jewish holiday yesterday. so getting around to patching it now.

thanks for the effort. I merged the cats stuff knowing their was a problem bug didn't have time to fix.

but as I have said to you many times before.
PR's w/o tests are useless. and waste MY time. in the future pls respect that.
 @jreback 

-1- pandas master [started failing](https://travis-ci.org/pydata/pandas/builds) exactly 16 hours ago, and this patch was targeting that _very exact_ fail (you know _one patch_ for _one fix_ thing). so not sure how exactly you worked it out couple of days ago

-2- in this case, there were tests already failing on travis build. so technically the tests were already there. you do not agree?!

as i told you this is just annoying and disrespectful that you do not care for the time/effort spent for a pr and close it with a "my own patch" argument. you may want to work on that.
 @behzadnouri not going to argue with you any more. Pls submit fully functional patches with tests / release notes / docs.

you contributions are appreciated, but I have noticed that you seem to be arguing (with not a lot of constructive criticism). Pls check that.
 @jreback 

for the record: [this](https://github.com/pydata/pandas/pull/8396/files) is the patch i had submitted, and [this](https://github.com/pydata/pandas/commit/df3224ffa05791179d6e3c546e1f917a9a2a81f0) is yours;

-1- contrary to what you said above:

> I already have a patch for this with some other code.

there is absolutely no other code.

-2- there are absolutely no new tests

-3- I also had extra code comments in my patch which you have removed them.

this is just wrong on many levels.
 @behzadnouri you are beating a dead horse.

I had this patch queued up since I had merged the original categoricals. It was supposed to be included but wasn't amended property.

get over it
