remove docker dependency on graphicsmagick

.docker/dockerfiles/latest/Dockerfile

@@ -2,10 +2,6 @@ FROM node:0.10
 
 MAINTAINER buildmaster@rocket.chat
 
-RUN apt-get update \
-&&  apt-get install -y graphicsmagick \
-&&  rm -rf /var/lib/apt/lists/*
-
 RUN groupadd -r rocketchat \
 &&  useradd -r -g rocketchat rocketchat \
 &&  mkdir /app  \