To support environments except bash/zsh( such as csh) 

Plenv requires bash and its document supports not only bash also zsh, so those have less readability, I think.

According to the POD, it depends on bash, However actually, it may be caused by that "$(plenv init -)" outputs codes in bash for `eval`.

So the new to do for us is just fix documents like bellow:

```shell
bash -c 'eval "$(plenv init -)"'

```
