Problems Installing Metasploit Framework on Ubuntu

I have encountered 3 problems while following this guide to installing Metasploit Framework on Ubuntu and Debian:

1) After installing proper version of ruby, there is a command given for installing Ruby libraries: sudo gem install bundler. When I typed that in and hit enter, it says command not found. After some researching on the internet, I decided to remove sudo and try again. So I've entered gem install bundler and it worked fine. Will this cause any problems?

2) After installing Metasploit Framework, there is a command given to install the required gems and versions:

cd metasploit-framework
bundle install

But when I tried that, I got this error:

rbenv: version '2.1.6' is not installed

I really don't know what to do next (But I moved on).

3) Probably due to the previous problems, in the end when I executed msfconsole command, it said

could not find rake-10.4.2 in any of the sources.

run bundle install to install the missing gems But the bundle install doesn't help either, because when I run that it says

Could not locate Gemfile or .bundle/directory

Any help would be appreciated.

EDIT: When I asked the person himself who had written the guide, regarding this, he told me to select the version of ruby I had just installed with either rebind or RVM before installing bundler, and then to run the bundle install. Could you please list the commands to do so? I am really new to Linux!