install_prereq: Update repositories before install on Debian systems

When to install packages the indexed local is more old of the
version of software on the repository they have been upgraded by security
update then get the package will give 404 not found.

The patch prevent by update local index to repository for aptitude before
install.

ASTERISK-25495 #close

Reporte by: Rodrigo Ramírez Norambuena

Change-Id: I645959e553aac542805ced394cac2dca964051fa

contrib/scripts/install_prereq

@@ -101,6 +101,7 @@ handle_debian() {
 		apt-get install aptitude
 	fi
 	extra_packs=`check_installed_debs $PACKAGES_DEBIAN`
+	$testcmd aptitude update
 	$testcmd aptitude install -y $extra_packs
 }