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. 
Thanks for asking @yggie 

## `nodejs` dependencies

There is an issue in Ubuntu 14.04 which prevents from installing dependencies:

    Some packages could not be installed. This may mean that you have
    requested an impossible situation or if you are using the unstable
    distribution that some required packages have not yet been created
    or been moved out of Incoming.
    The following information may help to resolve the situation:

    The following packages have unmet dependencies:
    nodejs : Conflicts: npm

Also this is from `dpkg -s nodejs`

    Package: nodejs
    Maintainer: Jérémy Lal <kapouer@melix.org>
    Version: 0.10.32-1chl1~trusty1
    Replaces: nodejs-dev, npm
    Provides: nodejs-dev, npm
    Conflicts: nodejs-dev, npm

I think this means that when installing `nodejs` package you automatically install `npm` and `npm-dev`

Error installing nodejs-legacy on Ubuntu 14.04

    Unpacking nodejs-legacy (0.10.25~dfsg2-2ubuntu1) ...
    dpkg: error processing archive /var/cache/apt/archives/nodejs-legacy_0.10.25~dfsg2-    2ubuntu1_all.deb (--unpack):
    trying to overwrite '/usr/share/man/man1/node.1.gz', which is also in package nodejs 0.10.32-   1chl1~trusty1
    Processing triggers for man-db (2.6.7.1-1ubuntu1) ...
    Errors were encountered while processing:
     /var/cache/apt/archives/nodejs-legacy_0.10.25~dfsg2-2ubuntu1_all.deb
    E: Sub-process /usr/bin/dpkg returned an error code (1)

`nodejs-legacy` is only about a symlink for `/usr/bin/nodejs` to `/usr/bin/node` and maybe not neccesairly, but if you want I could create it manually by issuing command: 

**Added in commit 47d0c21**
```sh
sudo ln -s `which nodejs` /usr/bin/node
```

Details about nodejs-legacy can be found [here](https://lists.debian.org/debian-devel-announce/2012/07/msg00002.html) and from package itself.
```sh
This package contains a symlink for legacy Node.js code requiring
 binary to be /usr/bin/node (not /usr/bin/nodejs as provided in Debian).
```

## `bundler` dependencies

Regarding the bundler dependency
```sh
Depends: ruby | ruby-interpreter
```

`ruby` depends on:
```
Depends: ruby1.9.1 (>= 1.9.3.194-1)
```

`ruby-interpreter` is virtual package

I don't want to install **systemwide** `ruby` only to install **one gem** - `bundler`, so I have moved installation of `bundler` after installation of `ruby` and install it as a gem into selected `ruby` version. This also ensures that `bundler` **is the newest version** available for used version of `ruby`

## Only removed `nodejs-legacy`

I have only removed `nodejs-legacy` because I couldn't get it to install on `Ubuntu 14.04`. `npm` is installed with `nodejs` package, and `bundler` is installed as a gem. Also I create a symlink to nodejs as node in user PATH.