Bump deployment target to iOS6

As iOS 7 released, there're some mobile analytics provider (e.g. Mixpanel) are moving their minimum iOS version support to iOS6, which results in some compatibility issue between `ARAnalytics` and those analytics SDKs.

`pod spec lint` would raise following errors:

```
[!] The platform of the target `Pods` (iOS 5.0) is not compatible with `Mixpanel (2.2.1)` which has a minimum requirement of iOS 6.0.
```