travis: add xapian-backports PPA

We really want to be testing with a more current version of xapian
than that available in Ubuntu Xenial

diff --git a/.travis.yml b/.travis.yml
index a2caf010fdec7ab8de6fadb7e5bb938099fa5858..f6d729f233ab0a61775a87cccdc79ff761565d59 100644 (file)
--- a/.travis.yml
+++ b/.travis.yml
@@ -4,6 +4,8 @@ dist: xenial
 
 addons:
   apt:
+    sources:
+    - sourceline: 'ppa:xapian-backports/ppa'
     packages:
     - dtach
     - libxapian-dev