Squashed 'rbenv/plugins/ruby-build/' changes from 4ef4ed4..498062c

498062c ruby-build 20140918
d646595 removed yaml from Ruby 2.2.0 preview1
de310f5 Merge pull request #637 from cosmo0920/2.2.0-preview1
997d14c released 2.2.0 preview1

git-subtree-dir: rbenv/plugins/ruby-build
git-subtree-split: 498062cd0d9ea632568f41a5c680a73f686fc4de
Merge commit 'ae4b062d95fa4eed25d522753aec8b5872d77f90'

Pull https://github.com/sstephenson/ruby-build to
`rbenv/plugins/ruby-build`.