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.
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:
And then you should be able to install the latest ruby version with:
rvm install 2.1