Update Buffer SDK submodule to include v1.1.1

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.