update default ruby version
update :rbenv_ruby_versions before building since it may be referenced.
1.0.0rc1
1.0.0rc2git
declare `RBENV_ROOT` in command line to reuse it for other users.
add `:rbenv_setup_global_version` to skip settings global version
add :rbenv_configure_options to pass `CONFIGURE_OPTS` to ruby-build
support invoking commands via rbenv.exec using sudo
skip installation if the requested version has been installed
release 1.0.0rc2

* fix using rbenv via sudo
* try building ruby only if it has not been installed
* skip setting global version if requested
1.0.0rc3git
fix in test case. use /dev/null as known_hosts to discard SSH host key.
style issue
try setting global even if it has not newly installed.
release 1.0.0
Merge branch 'release-1.0.0'
Merge branch 'master' into develop
1.0.1git