disabled assertions for deployment config

Fixes crash reported in issue #348.

Assertions should generally be disabled in release/deployment configs. 
The call to `[spinner stopAnimation:self]` with no matching startAnimation is safe as so doesn't need the assertion to block it in release.

[Some more info about why assertions shouldn't be present in release code](http://myok12.wordpress.com/2010/10/10/to-use-or-not-to-use-assertions/).