Set deployment target to 10.9

I've been travelling and haven't made any Time Machine backups in a while so have been holding off on installing Yosemite.

Setting the deployment target to 10.9 prevents this error on Mavericks:

```sh
dyld: lazy symbol binding failed: Symbol not found: _objc_readClassPair
  Referenced from: /usr/local/bin/sourcekitten
  Expected in: /usr/lib/libobjc.A.dylib

dyld: Symbol not found: _objc_readClassPair
  Referenced from: /usr/local/bin/sourcekitten
  Expected in: /usr/lib/libobjc.A.dylib

Trace/BPT trap: 5
```