centering search field on select2 plugin in admin area

I'm current working on ```2-1-stable branch``` in a project, and I noticed the search field on select2 plugin on admin area was not aligned, like here:

![before](https://f.cloud.github.com/assets/1538066/1303457/254e98da-3158-11e3-90cb-52bb61cbc055.png)

I pulled master branch to and ran the ```spec/features/admin/products/stock_management_spec.rb``` spec, and also noticed that was not aligned.

I made this little change on the css, and now it looks like this:

![after](https://f.cloud.github.com/assets/1538066/1303458/25596d46-3158-11e3-8591-05cb420ef51c.png)

I noticed this on a Chrome on Mac, and did this change using Firefox on Ubuntu.