Update "installing custom software" for "everyone has sudo"

Now that everyone has sudo, stop telling them to ask us to install stuff
for them.
Update the docs a bit more to make apt-get feel like the default
Merge pull request #3293 from circleci/desudo-installing-custom-software

Update "installing custom software" for "everyone has sudo"
Merge commit 'e4e0cd2e3513cff7a6c675f655662e4312e47b18'