Update _flexbox.scss

Notes:

* I don't think browser support is in a place where `$flex-legacy-enabled` should be false by default. Also, other Compass experimental variables are true by default in most cases. An "opt-out" policy seems better overall.

* I think 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` is the magical vertical-alignment property that has launched a thousand blog posts. (See: http://timhettler.github.io/compass-flexbox/tests.html)