If you get stuck with this strange error of uninitialized constant ActiveSupport::Dependencies::Mutex (NameError) don’t worry.
Just follow the steps below :-
gem update –system 1.5.3
once its installed successfully do gem list and see if you have any other versions of rubygems-update already installed. If so remove all the others apart from 1.5.3
Once you do that run the following command
The above command will show that 1.5.3 is installed. Do confirm the same by checking the gem version gem -v
Hope this helps. In case you have any query related to this do post a comment and I will get back ASAP