Add env parameter to rbenv::build #30

* replace `cflags` parameter with generic `env` parameter
** Example env parameters available
[here](https://github.com/sstephenson/ruby-build#special-environment-variables)
* add [puppetlabs/stdlib](https://github.com/puppetlabs/puppetlabs-stdlib) requirement
* update specs so tests pass
* update ubuntu Vagrant manifest for full example of new `env` parameter
* update compatibility metadata
Add ability to auto-update rbenv and rbenv:plugin #29

* `class { 'rbenv': latest => true }` will keep rbenv updated
* `rbenv::plugin { 'sstephenson/ruby-build': latest => true}` will keep
  ruby-build definitions updated