Bump blaze-layout to 0.2.3.

See:
https://github.com/EventedMind/blaze-layout/commit/27d2a0e67a2636b85668abd929af9fc64c818356.

- More helpful error message for {{yield}}.
- Allow {{> yield "footer"}} in addition to {{> yield region="footer"}}
- Same for contentFor
- @tmeasday, can you take a quick look?
- cc @avital