added graphicsmagick to install steps
added graphicsmagick to install steps

Sinar/popit,mysociety/popit,mysociety/popit,Sinar/popit,openstate/popit,Sinar/popit,openstate/popit,Sinar/popit,openstate/popit,mysociety/popit,mysociety/popit,openstate/popit,mysociety/popit