Add 1.9.3-p374
Merge pull request #269 from sorah/193p374

Add 1.9.3-p374
Invoke chmod for group or world writable directories only
ruby-build 20130118
Support selection of 'make' tool via environment variable
Merge pull request #272 from onibox/make-env-var

Support selection of 'make' tool via environment variable
Merge pull request #271 from nobu/master

No needs to chmod all directories.
Update examples
Allow plugins to run code before and after the installation process
`rbenv install` with no arguments defaults to `rbenv local`
Confirm reinstalling already-installed versions unless invoked with --force
Move MAKE default near the others
Ensure $MAKE is quoted
Document $MAKE
ruby-build 20130129
Add support for `RUBY_CONFIGURE_OPTS` and `RUBY_MAKE_OPTS`

These variables let you pass Ruby-specific configure and make options
that will not be passed to dependent packages like libyaml.