Update _flexbox.scss

Notes:

* I don't think browser support is in a place where `$flex-legacy-enabled` should be false by default. It gets situationally set to false if you're doing advanced flexbox stuff anyway, so an "opt-out" policy seems better overall.

* Support for `align-content` & `align-items` was mixed up. `align-content` only takes effect when there's line wrapping, so legacy support is out. `align-items` (`box-align` in the legacy syntax) is the property responsible for cross-axis alignment. (See: http://timhettler.github.io/compass-flexbox/tests.html#align-items)