I cannot install ruby new version. I stall the home-brew and ruby-build, both install successful. I type $ rbenv, it shows

...

install Install a Ruby version using ruby-build

...

then i stall version 2.3.1

`$ rbenv install 2.3.1`

it shows error

```
Zhimings-MacBook-Pro:~ zhimingjiang$ rbenv install 2.3.1
Downloading ruby-2.3.1.tar.bz2...
-> https://cache.ruby-lang.org/pub/ruby/2.3/ruby-2.3.1.tar.bz2
Installing ruby-2.3.1...
mkdir: /usr/local/var/rbenv/versions/2.3.1/lib: Permission denied

BUILD FAILED (OS X 10.11.5 using ruby-build 20160426)

Inspect or clean up the working tree at /var/folders/wf/tkkf34w562q8mqb59f43lnrh0000gn/T/ruby-build.20160703231044.15035
Results logged to /var/folders/wf/tkkf34w562q8mqb59f43lnrh0000gn/T/ruby-build.20160703231044.15035.log

Last 10 log lines:
/var/folders/wf/tkkf34w562q8mqb59f43lnrh0000gn/T/ruby-build.20160703231044.15035 ~
/var/folders/wf/tkkf34w562q8mqb59f43lnrh0000gn/T/ruby-build.20160703231044.15035/ruby-2.3.1 /var/folders/wf/tkkf34w562q8mqb59f43lnrh0000gn/T/ruby-build.20160703231044.15035 ~

```

I does't have path `/usr/local/var/rbenv`, I only have path `/usr/local/rbenv`

Additions (here is the tutorial i have watched
<https://www.youtube.com/watch?v=jx0NrIbQbzI>)