Merge branch 'master' of github.com:tokuhirom/plenv
Squashed commit of the following:

commit 7679195fe9132aebe7593cf7720826ad9007bf55
Author: WATANABE Hiroaki <hwat@mac.com>
Date:   Wed Apr 24 22:01:46 2013 +0900

    Remove the holding part.

commit 4a86c0289977980236eabe1140a644b97e786918
Author: WATANABE Hiroaki <hwat@mac.com>
Date:   Sat Apr 20 20:34:19 2013 +0900

    Add the force option to install