Switch to ruby version per project on rbenv

cd [project_dir]
echo "1.9.3-p362" > .rbenv-version

Example

$ cd ~
$ rbenv global 2.0.0-preview2
$ rbenv versions
  1.9.3-p362
* 2.0.0-preview2 (set by /Users/kengos/.rbenv/version)
$ ruby -v
ruby 2.0.0dev (2012-12-01 trunk 38126) [x86_64-darwin12.2.1]
$ cd [project_dir]
$ rbenv versions
* 1.9.3-p362 (set by /[project_dir]/.rbenv-version)
  2.0.0-preview2
$ ruby -v
ruby 1.9.3p362 (2012-12-25 revision 38607) [x86_64-darwin12.2.1]
$ cd ~
$ ruby -v
ruby 2.0.0dev (2012-12-01 trunk 38126) [x86_64-darwin12.2.1]