Remove Podfile.lock in demo project

Due to [the Cocoapods issue #1623](https://github.com/CocoaPods/CocoaPods/issues/1623), having the Podfile.lock in the demo project for the local pod cause `pod install` to fail with a cryptic

    Resolving dependencies for target `Pods' (iOS 6.0)
      - GDIIndexBar (from `../GDIIndexBar.podspec`)
    [!] Unable to satisfy the following requirements:
    - `GDIIndexBar (from `../GDIIndexBar.podspec`)` required by `Podfile`

Removing Podfile.lock solves this, and is not an issue as the local GDIIndexBar is the only pod referenced.