Fix deprecation error.

Without fix, this error message appears:

    bin/vim-ruby-install.rb:15:in `<main>': Use RbConfig instead of obsolete and deprecated Config.

    uninitialized constant Env::CONFIG
    Try 'ruby bin/vim-ruby-install.rb --help' for detailed usage.
Reformat installer.

Replace tabs with spaces. Ensure comments are properly lined up with code.