Fix "Build Active Architecture Only" build setting for iOS target

:warning: **Depends on Quick/Nimble#19** :warning:

As in the mentioned Nimble pull request, the `ONLY_ACTIVE_ARCH` ("Build Active Architecture Only") build setting should be set to `NO` for iOS targets, as per jspahrsummers/xcconfigs@ae4762d:

> If a library only built one architecture, then was used in a parent project that wanted all of them, things asplode.

This should fix #111, i.e. phantom "unresolved identifier" errors, but requires Quick/Nimble#19 to be merged as well (and an updated submodule).