buildserver: install gettext from jessie-backports

In order to install a package from jessie-backports, apt-get has to be told
to get it from there rather than the main archive.  It will not use
jessie-backports by default even if it is added as an apt source.

closes #224

buildserver/provision-apt-get-install

@@ -40,7 +40,8 @@ packages="
  expect
  faketime
  flex
- gettext
+ gettext/jessie-backports
+ gettext-base/jessie-backports
  git-core
  git-svn
  gperf