re-add apt pinning to unit test on puppet 2.7

Author: bmjen

.fixtures.yml

@@ -1,6 +1,8 @@
 fixtures:
   repositories:
-    apt: "https://github.com/puppetlabs/puppetlabs-apt.git"
+    apt: 
+      repo: "https://github.com/puppetlabs/puppetlabs-apt.git"
+      branch: "1.8.x"
     stdlib: "https://github.com/puppetlabs/puppetlabs-stdlib.git"
     firewall: "https://github.com/puppetlabs/puppetlabs-firewall.git"
     concat: "https://github.com/puppetlabs/puppetlabs-concat.git"