Submodule Update: Update Buffer to 1.1.1

v1.1.1 includes the following updates...

- Resolved issues related to background vignette not covering the whole view in some cases.
- Tweaked presentation animation to mimic Twitter, Facebook composers more closely.
- Sheet now animates status bar style to UIStatusBarStyleBlackOpaque when displaying sheet. Animates back to previous style when dismissing sheet.
- Updated various image resources.

Also removed UIActivity references to allow iOS 5 support in ShareKit.