Update tests to expect align-items: stretch as default instead of flex-start

I'm submitting this for review: it does not update the JS tests so it probably shouldn't go in. I just updated the tests that were failing when I changed the defaults based on reading http://css-tricks.com/snippets/css/a-guide-to-flexbox/ for expected behavior. I'm pretty sure I got the expected behavior right but I want to make sure we're not reducing test coverage by making these changes. I'm not sure how you all decided what tests to put in in the first place so I'd like to get your thoughts.

Re: JS tests, I'm really not sure what's going on there. Making this change causes 50 tests to fail:
![image](https://cloud.githubusercontent.com/assets/594924/4554343/8bab6118-4eac-11e4-91c5-1f55416b875e.png)