install software-properties-common in order to use apt-add-repository. …

…resolves dnschneid#2520

targets/mate

@@ -2,7 +2,7 @@
 # Copyright (c) 2016 The crouton Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
-DESCRIPTION='Installs the mate desktop environment. (Approx. 150MB)'
+DESCRIPTION='Installs the mate desktop environment. (Approx. 205MB)'
 HOSTBIN='startmate'
 CHROOTBIN='crouton-noroot startmate'
 . "${TARGETSDIR:="$PWD"}/common"
@@ -18,9 +18,10 @@ if release -le precise ; then
 fi
 
 if release -eq trusty && [ "$ARCH" = 'amd64' -o "$ARCH" = 'i386' ]; then
-    sudo apt-add-repository ppa:ubuntu-mate-dev/ppa
-    sudo apt-add-repository ppa:ubuntu-mate-dev/trusty-mate
-    sudo apt-get update && sudo apt-get upgrade
+    install software-properties-common
+    sudo apt-add-repository -y ppa:ubuntu-mate-dev/ppa
+    sudo apt-add-repository -y ppa:ubuntu-mate-dev/trusty-mate
+    sudo apt-get update
 fi
 
 install mate-desktop-environment