doesn't make some simple yet very important checks before starting the installation process (e.g need permission when using --sytem flag)

[Mizou04]

Description

Steps To Reproduce

Steps to reproduce the bug:

1. $ ruby-install ...
2. ???

Expected Behavior

Actual Behavior

Environment

$ ruby-install --version
...
$ uname -a
...
$ cc --version
...
$ openssl version
...

[Mizou04]

I have a fix for this issue

[postmodern]

@Mizou04 feel free to submit a PR. I'm guessing an extra if [[ ! -w "$install_dir" ]]; then could be added to bin/ruby-install.

[Mizou04]

actually I modified directly the function responsible for parsing the options so it doesn't need to go the whole stack down to bin/ruby-install, is that ok?