Fix incorrect node[:ruby_build][:prefix] and node[:ruby_build][:bin_path]

Currently `node[:ruby_build][:prefix]` and `node[:ruby_build][:bin_path]` gives us wrong paths, which are `/opt/rbenv` and `/opt/rbenv/bin/rbenv` respectively. As a consequence, the genrated `/etc/profile.d/rbenv.sh` contains a wrong PATH value as following:
```sh
export PATH=$RBENV_ROOT/bin:/opt/rbenv/bin/rbenv:$PATH
```
where there exists `/opt/rbenv/bin/rbenv` instead of `/opt/rbenv/plugins/bin`. With this patch, the attribues would be fixed to `/opt/rbenv/plugins/ruby_build` and `/opt/rbenv/plugins/ruby_build/bin` as we expected.

I haven't managed to run the vagrant tests becase I got this error:
```
undefined local variable or method `use_inline_resources' for #<Class:0x0000000238b268>
```
but I've checked it in a in-house wrapper cookbook instead.