Use system to determine clipboard on linux.

- system() returns boolean, that's what we need here
- redirect stderr to /dev/null to avoid ugly error messages

Signed-off-by: ushi <ushi@porkbox.net>
Redirect output to /dev/null in the which call

Signed-off-by: ushi <ushi@porkbox.net>
File should only be accessible by the owner.

The only user with access to your clipboard file should be you.

Signed-off-by: ushi <ushi@porkbox.net>
update project meta files
add missing require "rbconfig"
Merge branch 'master' of https://github.com/ushis/clipboard into ushis-master
small tweaks for detecting xclip/xsel