setup: rbenv: add "rbenv local" while installing ruby

Run "rbenv local 1.9.3-p0" before finishing the ruby installation.
This makes sure the 1.9.3-p0 will be used during the Octopress installation.

Signed-off-by: Mark Zhang <super119@139.com>
Merge pull request #14 from super119/setup-rbenv

setup: rbenv: add "rbenv local" while installing ruby