fix GetCached impl

Fix for rustc 0.12.0-pre-nightly (0e784e168 2014-09-16 23:26:11 +0000)
Error:
src/lib.rs:116:1: 116:52 error: the parameter type `R` may not live long enough; consider adding an explicit lifetime bound `R:'static`...
src/lib.rs:116 impl<E: Extensible, R: Clone> GetCached<R> for E {}
               ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
src/lib.rs:116:1: 116:52 note: ...so that the parameter `R`, when instantiated with `R`, will meet its declared lifetime bounds.
src/lib.rs:116 impl<E: Extensible, R: Clone> GetCached<R> for E {}
               ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
error: aborting due to previous error