Don't update cache on yum search.

By default, `yum search` will update the repositories' information.
Pacman does not do this, and requires you to do an -Sy operation first.
Since we've implemented -Sy for rpm-based systems, we can mimic that
behavior more closely by forcing yum to not update the cache when
searching (which is also faster).
Merge pull request #14 from xiongchiamiov/dont-update-cache-on-yum-search

Don't update cache on yum search.