Failed install requires manual cleanup, as rbenv does not detect if it's sucessfully installed.

[ronabop]

https://github.com/ronabop/rbenv-cookbook/blob/master/libraries/chef_mixin_rbenv.rb

lines 59-61:

      def rbenv_installed?
        out = shell_out("ls #{rbenv_bin_path}/rbenv")
        out.exitstatus == 0
      end

Problem:
If the URL provided for the install is not available (for example, flaky internet, behind a corporate firewall), the first invocation of this cookbook creates an rbenv directory, and then fails to :sync. Subsequent invocations of the cookbook see that extant directory, and assume that rbenv has been installed, without needing to :sync, and fail.

PR #131 created.