set iOS 8 as minimum to allow App Store submission

App Store submission currently rejects binaries that include ObjectMapper because dynamic frameworks must have a deployment target of iOS 8 - ObjectMapper currently deploys to iOS 7.1.

When building a linker warning pops up that seems to hint at this:
```sh
ld: warning: embedded dylibs/frameworks only run on iOS 8 or later
``` 

And it turns out this is fatal at submission time, the App Store will reject binaries that include dynamic frameworks if those frameworks don't have a `minimumOSVersion` of iOS 8 or later. This isn't unique to ObjectMapper, other projects that have run into this and needed to support iOS 7 have elected to go to a static framework, see realm/realm-cocoa#900.