Document that puppetlabs-apt versions less than 2.x are required

Document that this module currently requires puppetlabs-apt versions less than
2.x, and test against 1.8.x

.fixtures.yml

@@ -1,7 +1,9 @@
 fixtures:
   repositories:
     "stdlib": "git://github.com/puppetlabs/puppetlabs-stdlib.git"
-    "apt": "git://github.com/puppetlabs/puppetlabs-apt.git"
+    "apt":
+      repo: "git://github.com/puppetlabs/puppetlabs-apt.git"
+      ref: "1.8.0"
     "portage": "git://github.com/gentoo/puppet-portage.git"
     "chocolatey": "git://github.com/rismoney/puppet-chocolatey.git"
     "epel": "git://github.com/stahnma/puppet-module-epel.git"

README.md

@@ -447,7 +447,7 @@ it is installed.
 
 This modules uses `puppetlabs-apt` for the management of the NodeSource
 repository. If using an operating system of the Debian-based family, you will
-need to ensure that it is installed.
+need to ensure that `puppetlabs-apt` version 1.x is installed.
 
 If using CentoOS/RHEL 5, you will need to ensure that the `stahnma-epel`
 module is installed.