Added a check in Makefile to see if composer is installed

Fix #2498

scripts/install.sh

@@ -2,6 +2,8 @@
 # You can execute this file to install wallabag
 # eg: `sh install.sh prod`
 
+command -v composer >/dev/null 2>&1 || { echo >&2 "wallabag requires composer but it's not installed (see http://doc.wallabag.org/en/master/user/installation.html). Aborting."; exit 1; }
+
 ENV=$1
 TAG=$(git describe --tags $(git rev-list --tags --max-count=1))