If you are doing a fresh install jump to step three! It will save you the hassle of doing it later. I had to do this to make my travisCI much easier to deal with on different machines.

Option 1: Change the permission to npm’s default directory

1. Find the path to npm’s directory:

    npm config get prefix
   

   For many systems, this will be /usr/local.

   WARNING: If the displayed path is just /usr, switch to Option 2 or you will mess up your permissions.

2. Change the owner of npm’s directories to the name of the current user (your username!):

    sudo chown -R $(whoami) $(npm config get prefix)/{lib/node_modules,bin,share}
   

   This changes the permissions of the sub-folders used by npm and some other tools (lib/node_modules, bin, and share).

This worked for me but there are other options. On the npm docs site.