Improve and simplify dotfiles

- Add Gemfile to install gems;
- Remove stuff for erlang, android, python, scala and racket;
- Add and remove several apps;
- Move dotfiles folder under $home;
- Update ruby and rbenv;
- Use symbolic links instead of file copies;
- Change font from Inconsolata to Ubuntu Mono (has Esperanto
  supersigno);
- Invert order of files on diff script;