Add sudo to linux instructions

[jonnymccullagh]

Just added a few commands to the linux install instructions to try to avoid permissions errors and cloning into wrong directory

[skylarmt]

cd ~ && should be cd ../ &&, otherwise it'll dump everything in the user's home folder.

[jonnymccullagh]

cd ~ && should be cd ../ &&, otherwise it'll dump everything in the user's home folder.

I thought it would be better to stick to the home folder for both git clones rather than mixing them and "cd .." has the potential to end up somewhere unknown if we don't know the starting point. We could add:
rm -rf ~/woff2 ~/sfnt2woff-zopfli
to tidy up.

Anyway we also needto install the package ruby-dev too.