Multiple sortable columns and sorting on associated resources

Hello,

this pull is a superset of #1766, that uses `meta_search`'s [`meta_sort`](https://github.com/ernie/meta_search/#sorting-columns) to implement sorting on multiple columns (#909), and also sorting on associated models' columns.

The only thing I don't like about this implementation discriminates between [virtual and concrete columns](https://github.com/ifad/active_admin/commit/a7bdb84b38c326b901c447f68e5cce388d298836#L0R221), by looking at the `.select_values`.

Anyway, specs & features pass (this is my first experience with cucumber - sorry if they look bad), and there's also an unrelated improvement to the tests set up, in which instead of using Rails 3.2.0 by default, the latest version is [fetched from the `.travis.yml`](https://github.com/ifad/active_admin/commit/b8978ecbfdb970bda3db9c3b2ed3bb960d96e7d1) configuration file.

Enjoy!

cc: @npj - we'll soon be able to remove that monkeypatch :beers:
cc: @Daxter - here we go! :dancers: 
cc: @zzeni - I've taken your suggestion into account, thanks! :+1: