Do apt-get update before installing in pre-install script

git-svn-id: https://humgenprojects.lumc.nl/svn/mutalyzer/trunk@384 eb6bd6ab-9ccd-42b9-aceb-e2899b4a52f1

extras/pre-install.sh

@@ -20,6 +20,7 @@ COLOR_END='\033[0m'
 
 echo -e "${COLOR_INFO}Installing packages with apt${COLOR_END}"
 
+apt-get update && \
 apt-get install -y \
   mysql-server \
   python \