Add support for rbenv existing users

I'm using rbenv and I want to contribute to AgileVentures, thus I have changed the setup scripts, so that I and possibly other developers don't fall into problems with having both rvm and rbenv installed and prevent conflicts between them.

* Support is only for existing rbenv users, so if one don't have it installed my changes to setup script does nothing to them.
* Also there is an issue I couldn't resolve - installation of Ruby 2.1.1 on Ubuntu - there is a readline.so problem preventing Ruby to compile and I have statically made a decision for user to install Ruby 2.1.2 instead which compiles just fine.
* One more thing - Installation of bundler as a gem, because otherwise I would have to install older Ruby systemwide first (like 1.9.3), so it could install bundler, but I will not use it after the installation. Nodejs package for Ubuntu 14.04 alreadey contains npm and nodejs-dev, and I have changed this, enabling installation of packages on Ubuntu 14.04 - otherwise it won't start, but will execute other commands - this is not what installation script should do.