Fix note from 'R CMD check git2r'

* src/libgit2/cache.c (cache_evict_entries): Replace 'rand' with R's
  'runif' to fix note "Compiled code should not call entry points
  which might terminate R nor write to stdout/stderr instead of to the
  console, nor the C RNG"

Signed-off-by: Stefan Widgren <stefan.widgren@gmail.com>