prune installation directories that no longer exists from `make install`

```
% find $DIRS -type d
```

Before prune:

```
find: `etc': No such file or directory
find: `lib': No such file or directory
bin
find: `sbin': No such file or directory
share
share/ruby-install
share/ruby-install/mruby
share/ruby-install/jruby
share/ruby-install/rbx
share/ruby-install/maglev
share/ruby-install/ruby
share/man
share/man/man1
```

After prune:

```
bin
share
share/man
share/man/man1
share/ruby-install
share/ruby-install/jruby
share/ruby-install/maglev
share/ruby-install/mruby
share/ruby-install/rbx
share/ruby-install/ruby
```