Problem

You are trying to install the new ruby in your ubuntu rvm installation with rvm install 2.1 but you are getting the error:

Branch origin/ruby_2_1_0 not found.
There has been an error while checking out branch ruby_2_1_0.
Halting the installation.                               
There has been an error fetching the ruby interpreter. 
Halting the installation.

Solution

If you are careful and you notice a warning message as:

Warning, new version of rvm available '1.25.26', 
you are using older version '1.24.8'.

then you would need to update your rvm installation to the latest one with:

rvm update

And then you should be able to install the latest ruby version with:

rvm install 2.1