Add travis config for testing against multiple databases

[fschwahn]

As discussed in #76, it's a good idea to test against the most commonly used databases (ie. mysql, postgres, sqlite). This pull request adds support for that. Some notes:

• Added database.yml with credentials. These credentials are the default from travis ci (for mysql, postgres). If no database is configured it still defaults to sqlite, meaning for existing developers nothing should change.
• Some tests were failing in mysql / postgres because they tested database specific behaviour (mostly order of nulls / default order). I did the minimum amount of work to get things passing, which in some cases meant flat out removing assertions.
• I added memoization for now and yesterday (see diff), because the test could fail in slow environments (like travis).
• I added an allow_fail directive for rbx-2, because it won't install on travis right now. capistrano did the same things recently because rbx support is so flaky.
• I added a cache for bundler which should speed up builds (I saw devise doing this and thought it couldn't hurt).

[brendon]

Thanks @fschwahn :)

[brendon]

Just noticed this failure: https://travis-ci.org/swanandp/acts_as_list/jobs/170359139 and one other on that test run. Would you mind investigating timecop as an alternative for freezing the time for comparison?